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Quadratic Functions

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In the two months between when they hooked up at that party and when they went home for the holiday break, Lydia, Derek, Erica and Stiles figured out a few things together:

1. They did not like threesomes, kind of at all. One-on-one time was great—essential, actually, for their relationship to keep humming—and being all together had to happen at least one night a week. But when there were just three of them it was like the missing person haunted them, and they felt unsettled. When it was Derek who was running behind, usually because he was stuck up in the chem labs waiting for something to happen, they sent him sexts and naked selfies of them in his bed but didn't touch each other (much) until he got back, flushed and hard and ready to fuck.

2. Having the poly conversation with their parents was trickier than having the bi conversation had been. They all just seemed confused about "how does this work, exactly" as the Sheriff said. But then they had a big dinner, since it turned out Derek and Erica only lived a few towns away, and apparently seeing the four of them together just made it make sense for people.

3. Their relationship started a completely insane rumor mill at their college which they did nothing to stop because they found it either insipid (Derek and Lydia) or hilarious (Stiles and Erica). Not that they could do much about it, anyway. Certainly it increased the listenership to Stiles and Erica's radio show, where they flirted and sent out dedications to Derek and Lydia who often worked together in Derek's office until it was over, and then Stiles drove them all back to Derek's.

4. Their birthdays were all in a row in the spring, two before and two after the break. They planned to spend the break on campus anyway, because Derek had to work and neither Stiles nor Erica had the money to travel anyplace all that cool anyway. So Lydia worked on some project and Erica and Stiles basically took over the radio station. It was all good.

Planning for their birthdays, though—that took a little more doing.

It started because when they asked Stiles what he wanted for his birthday he said, "Sex." Which, being cash-strapped students, was something they could give him.

The night before his birthday, Stiles and Scott hit a small sports bar that wouldn't look too closely at their fake ID's. Allison's present was to play chauffeur, and she even dressed up for the occasion in a suit with brass buttons down the jacket and a hat. Allison had a lot of hats, and Stiles suspected the entire driving and outfit thing was just so she could wear one. Of course she looked super hot, because Allison Argent could rock some menswear.

Scott and Stiles mostly had pitchers of Bud because sure, there was a time to sit and sip whiskey and learn to like it, but that wasn't in a bar that was enough of a dive to be soft on underage drinking. Besides, cheap beer went better with extra spicy hot wings and mozzarella sticks. They mostly just goofed around, though Scott got sentimental at one point about something that happened when they were six and they had to hug it out.

Stiles wasn't even that drunk when Allison dropped him off at Derek's place—drunk enough that he shouldn't be driving, sure, and he was a little more stumbly than usual, but he still knew his own name, which was more than he could say for his birthday the year before. Maybe he really was growing up, or something; he even remembered to chug a giant glass of water and take a couple of aspirin before he crashed.

It wasn't exactly tragic to be woken up at six-thirty, though it helped that it was a fresh-from-the-shower Derek doing the waking and that he was carrying one of his high-powered smoothies.

"Oh my god," Stiles said, making grabby hands. He slurped down a big gulp and could feel all the good things in the smoothie flowing into his bloodstream and making him feel a little more human. "Raspberry peanut butter, you are the best boyfriend ever."

"Happy birthday," Derek said, smiling, but also not quite making eye contact.

Stiles just kept sipping because Derek generally got there if you didn't rush him.

And after a few moments of silence he finally asked, "So, wanna fuck me before I head to class? I'm clean, obviously, and I got myself ready so you can just lie back—"

"I'm definitely not just lying back," Stiles replied, then sucked down the rest of the smoothie as fast as he could. "You arrange yourself however, and I'm going to brush my teeth and pee and come right back. Also?"

"Yeah?" Derek asked.

"Seriously you have to stop acting like asking me to fuck you is this huge request. I know I'm impatient sometimes but I love fucking you. Okay?"

Derek took a deep breath, but he was nodding. "Okay."

Stiles didn't rush the brushing because under the layer of smoothie his mouth basically felt like the carpet on the floor of the jeep, ugh, and Derek deserved to kiss something nicer than that. By the time he returned to the bedroom, Derek had taken his boxers back off and was laying on his back, spread out in the middle of the bed. Stiles knew there would come a time when that sight didn't make him want to gasp, but he was glad they weren't there yet.

Derek had even gotten out the condom and insisted on rolling it onto Stiles and lubing it up himself, but after that Stiles took over, and Derek was pliant and gorgeous under him and so, so responsive. Stiles couldn't decide if his favorite part was the fucking, or watching Derek get fucked, or making him come, or coming himself. But it might have been laying in bed after, watching Derek get dressed, knowing that everyone up in the chem labs would be getting a blissful fucked-out Derek thanks to Stiles. He wasn't into marking his lovers—that was more Derek's thing—but he couldn't help feeling pretty damn proud of himself. Fucked-out was a good look on Derek.

And Stiles? He rolled over and went back to sleep.

By the time Erica showed up, a little after noon, Stiles was sitting on the couch in a t-shirt and boxers, building stuff in Minecraft. He was so not going to class on his birthday, no sirree.

"Happy birthday," she said, plonking a bag on the coffee table.

Stiles looked in the bag—chicken fingers and curly fries, nice—but Erica was walking right into the bedroom.

"What?" he asked.

She pulled her hair out of its bun and looked over her shoulder. "Wanna fuck me doggy-style in front of the mirror?" she asked.

He jumped up. "There's more than one answer to that question?"

Erica was already out of her little dress and was kicking out of her shoes and slipping off her panties. "Want these? I have another pair around here someplace."

"Again, there's more than one answer?" He grabbed for them, but she moved her hand.

"Kiss me first. I'm not that easy."

He paused and looked at her.

"Okay, I am for you, especially on your birthday, but I'd still like a kiss, please."

"Come here," he said, and laid one on her. She tasted like cinnamon gum.

"Thanks," she said, handing him the panties. Then she got on her hands and knees in the middle of the bed and wagged her ass at him as he went to the nightstand to get a condom. "Need some help?"

"Give a man a minute!" But who was he kidding, really. It had been almost six hours since he'd fucked Derek, he hadn't masturbated since then, and here was his super hot blonde girlfriend begging for it. Her panties smelled awesome, too. They were so fucking wet, which gave him the bright idea to rub them against his hardening cock, and that totally did the trick. He tossed them down on the bed and knelt behind her. "Ready?" he asked.

"Do it," she said, bracing herself and smiling up at him in the mirror.

He lined himself up and slid into her. "Wow, your tits look amazing like this."

"Yeah?" she asked, looking down at them still in her bright blue bra and then back up in the mirror. "Cool. Now what are you waiting for? Go, go, go!"

They went at it so fast that when they were done the chicken was still warm.

Stiles called his dad, and then spent the rest of the afternoon watching some baseball documentaries that Derek had recommended and dicking around on the internet. He was also vaguely wondering what Lydia was planning. She'd texted that she was bringing him dinner but what would the sex present be? Would there even be one? You could never be sure with Lydia, or at least, Stiles never was.

She came by before her evening class with salads. But she left them on the table and sat down on his lap.

"Hey," he said, putting his arms around her.

"Happy birthday," she said, and kissed him forcefully, her hands wrapped around his neck. She wiggled her ass in his lap and okay, so it was going to be like that.

Which was more than fine. He got off on her using him for her pleasure, and she knew it.

She pulled back. "Doesn't take much to get you ready, does it?"

"Not for you," he said.

"Not for any of us, I've heard. At least today."

Because of course Erica had totally texted Lydia, or talked to her, or whatever. He wondered if they'd talked to Derek, too. "Is that a problem?" he asked.

"Not at the moment." She stood up and unceremoniously pulled out his cock, rolling on the condom that she'd had in her skirt pocket. Then she pushed off her panties and sat down on it. "No problem at all."

And he had to laugh at that, because it was so Lydia, and far from the first time she'd done this. "You driving?"

She nodded. "Just put your hands on my waist."

But usually when she did this she closed her eyes, concentrated on what she was doing, was kind of selfish about it. Which was the whole point really, and Stiles liked that. This time, though, she didn't take her eyes off him, not for a second. Her hands went from his shoulders to cupping his face, and it was so intimate that he couldn't bring himself to say a single word. Even when he felt that he was going to come soon—and he really wanted her to come first—it was all he could do to thrust up into her, as a little warning.

"It's okay," she whispered, smiling at him. "Come for me."

He did, his eyes opening wider, and she gritted her teeth and maybe she was coming, too, and it was … a lot.

They caught their breath, made out some more before Lydia slipped off him and put her panties back on. She handed him his salad, full of cheese and croutons, tossed with extra french dressing and therefore a little soggy, which was just how he liked it. She settled on the other end of the couch but kept her feet pressed up against his thigh, and he lay one arm down over her ankles. Lydia wasn't one to cuddle, especially after things got intense.

They ate in silence, quickly; Lydia had a class to get to, after all. When Stiles finished he caressed her ankle, affectionately. "So this has been a excellent birthday," he said.

Lydia laughed. "It's not over yet," she said. "You don't think there's going to be a big finish?"

"Really?" he asked. Because he'd hoped—was maybe hinting around for info a little bit there—but it had already been pretty great.

She looked at her reflection in the TV. "God, you've totally messed up my hair," she said, pulling it into a ponytail.

Which, score one for Stilinski.

Then she stood up and grabbed her bag. "Derek left something for you in the bathroom," she said, and kissed him before walking out the door.

"More presents for me," Stiles said. "Awesome!"

Sure enough there was a gift bag in the tiny linen closet and what was in it—well, after everyone wanting the D all day, it looked like the big finish was going to be Stiles getting seriously fucked. He wondered what the girls would be doing.

When Derek came back Stiles was still in the shower, because … ugh.

"All clean?" Derek asked, stripping and climbing in.

"There'd better be rimming in my future," Stiles said, poking him in the chest. "Because I popped a boner again using that thing."

Derek blinked at him, and then began running his hands gently along Stiles's arms, his go-to soothing maneuver, and Stiles went with it because seriously. "I'm sorry the douche still freaks you out. Thanks for using it."

Stiles shrugged. "It's polite, I guess. I just—what if it's like, the first step to becoming some giant enema freak?"

"Given how you're feeling right now, I think that's pretty unlikely. You were probably just thinking about getting fucked later."

"Yeah," Stiles said, sighing. "Thanks. Sorry, um, welcome home." He leaned in for a kiss.

"It's all right," Derek said. "Anyway, you like having things in your ass." He slipped one finger into Stiles's cleft, illustratively.

"Is that my present, things in my ass?" Stiles asked, leaning into Derek's hand.

Derek didn't give him an answer, of course. "Get out of the spray so I can clean up."

"I can help," Stiles replied, grabbing Derek's sponge as they traded places.

Once they were dry, Derek made good on his promise of rimming, laying Stiles out on the bed and using his fingers and tongue and a lot of lube.

"Fisting?" Stiles asked.

"No," Derek said. "I wouldn't surprise you with that."

"But you want to?" Stiles asked, turning to look at him.

Derek paused, looking up and making eye contact. "We can work up to that, if you want to."

"Let's put it on the list," he replied.

Stiles was quiet after that, except for the moaning and the "yesses" and the "fuck, right theres." He was floating out on a pretty great bliss stream, sinking bonelessly into the bed, when he heard the ladies come home. He was just about with it enough to turn his head toward the bedroom door.

Lydia and Erica were leaning in, all sexy conspiratorial smiles. "Got him ready?" Erica asked.

"Just about," Derek replied. Because he was a dick, he stroked Stiles's prostate with the tip of a finger, making Stiles moan and twitch.

"Fuck," Stiles said, and he wondered if he should be annoyed at Derek showing off his effect like this, and then decided he couldn't be bothered. It wasn't like the ladies didn't know.

"Good," Lydia said. "We're taking a shower."

"Can I watch?" Stiles asked.

"No!" Erica said.

Which was okay because Stiles wasn't sure he could stand up all that well anyway. It was hard enough to move around on the bed as Derek wanted him to, but by the time Lydia and Erica returned, Stiles and Derek were leaning against the head of the bed, Stiles's back against Derek's chest. Erica was naked but Lydia had on a pair of boxer shorts that looked puffy and strange compared to her usual tiny lacy panties.

"All set?" Derek asked.

Erica nodded. "Ready for your present?" she asked.

"Yes?" Stiles replied, because of course, but also, he couldn't imagine what she and Lydia were grinning about.

Until Lydia dropped the boxers to reveal a strap-on. It was bright green, not upsettingly enormous but definitely big enough to get the job done, and it looked like it belonged, a natural contrast with her freckled skin. "Happy birthday," she said.

"Oh my god," Stiles said.

Erica lay on her back at the foot of the bed, and not for the first time Stiles was thankful they'd all chipped in to get Derek a bigger mattress. "Like it?"

"Yes!" Stiles's voice cracked, but he couldn't even be embarrassed about that.

"Good," Lydia said, kneeling between Erica's spread legs. "Wouldn't want all our practicing to go to waste."

"P-practice?" Stiles asked, because she couldn't possibly mean—but she did! She did, Lydia was totally pushing that strap-on into Erica's wet pussy. "Oh my god."

"I hope you don't mind," Erica said, her breath hitching just slightly as Lydia fucked her. "You know how Lydia is. She doesn't like to do anything unless she can do it well."

"It wouldn't be a present if there was fumbling," Lydia said. "We thought it would be okay if you watched."

"Totally okay," Stiles said. "Abso-fucking-lutely okay."

Stiles could feel Derek chuckling. "Also Erica makes the best lube."

"It's true," Erica said.

"Two birds and all," Lydia said. "And I think I got the hang of it, anyway."

"I would agree," Stiles said. His brain was still having trouble comprehending what was happening; he'd never spent much time watching pegging porn himself. Though now that he thought about it, Erica had shown him more than a few, both straight and lesbian, and he'd been appreciative. He just hadn't realized that was a test of concept.

Erica, interestingly, mostly just looked like she looked when she was getting fucked by Derek or by Stiles himself: happy to be along for the ride, one hand lazily stroking her clit while the other sat on the waist of her partner.

"I can help with that," Stiles said. "If you want."

They both looked at him. "But it's your birthday," Lydia said.

"I'd like to."

He sat up, his body feeling a little more under his control than it had been earlier, and slid his legs around so that he was kneeling over Erica. But then he felt the absence of Derek, so he turned around. "You should, um ..."

"What, Stiles?" Derek asked. "What do you want?"

Stiles took a breath. It wasn't like he had a problem asking for things on a regular day, but he was a little overwhelmed, understandably. "Fingers?"

Derek smiled, but he was sitting up, too. "You mean, the part where you like having things in your ass?"

"Shut up," Stiles said, but he got what he wanted. Derek wrapped one arm around Stiles's waist as the fingers of his other hand worked slowly in and out, and it was all strangely comforting.

Satisfied, he turned back to Erica and Lydia. He slid his left hand down her stomach to join her fingers at her clit, just above where Lydia was thrusting in and out. He could feel Derek's breath on his shoulder, and knew that his boyfriend was watching, too.

"Go ahead," Erica said, relinquishing her clit to him, and he gladly took over, stroking it between his fingers the way he knew she liked. Up close he could see the shine of Erica's wetness on the dildo, the way it parted the folds of her skin.

He turned to Lydia. "Can I ..."

"Sure," she said, nodding but not looking up.

He put his other hand on Lydia's back, relying on Derek to keep him upright and balanced, and moved it slowly down to the strap of the harness, just above the curves of her behind. Her hips were thrusting, steady, and it felt like when Derek fucked him, that easy rhythm.

"Do you like it?" Stiles asked. "Does it—can you come from it?"

"I haven't figured out how, yet," she said, sounding a little regretful. "But I still like it quite a lot." She grinned.

He smiled back. "Top."


"Hey, who fucked who earlier?"

She turned to him and raised an eyebrow. "I'm going to let you think about that one."

"Close," Erica said, and they turned their attention back to her. Stiles moved his fingers faster against her and Lydia sped up, too. Erica was staring at Derek, just over Stiles's shoulder, which was smart—Derek could absolutely make you come just with his eyes. It was like, his superpower.

And come she did, moaning and muscles clenching, so fucking gorgeous the way she gave herself over to her orgasms, completely lost in it. Stiles always felt honored that she trusted them with that moment when nearly all of her walls came down.

"Mmm," she sighed, and turned to Stiles, smiling. "Your turn."

Lydia sat back on her haunches, the dildo sliding out of Erica easily, little trails of her wetness stretching out between.

Stiles licked his lips. "Next time you fuck her, I'm licking that thing clean."

"Not if I get to it first," Derek said. "Come on." He pulled his fingers out of Stiles and slid them both backwards to where they had been, backs up against the wall behind the bed. Then he put a hand behind each of Stiles's knees and spread his legs wide.

"Wow," Erica whispered.

Lydia showed no hesitation. "Ready?"

Stiles licked his lips. "So ready. Way fucking past ready."

She nodded and then she was lining herself up and pushing into him.

"Slow and steady," Derek said. "We've got all night."

"No no no no no," Stiles said. "No slow, why slow?"

Derek shook his head. "Don't listen to him. I don't."

"Cause you're a—" Stiles stopped, wincing at a sudden twinge of pain, but stubbornly refusing to cry out and show that Derek was right.

"She can't feel like I can, so try to be patient," Derek said, but he softened his words with a nuzzle into Stiles's neck and a kiss on the temple.

And so Stiles kept his mouth shut, relaxed and trusted in the process, even if they were getting there more slowly than he would have liked. Watching Lydia was a good reward, as concentration always made her look sexy but concentration on sex was downright devastating. Erica loved seeing guys getting fucked anyway, so she was all over it. It should have made him feel self-conscious, with Derek spreading him wide open and everyone so focused on his asshole, but it felt too good for him to worry about it. Besides, it wasn't like they hasn't all already seen it, fingered it, kissed it, watched Derek fuck it.

Eventually Lydia was buried in him up to the hilt. "All right?" she asked.

He nodded and put his hands on her ass. "Let's do this."

Lydia rocked back and forth, getting a rhythm going. It was different in plenty of ways—no balls slapping against his ass, smooth skin under his fingers instead of soft hair, the straps themselves—but it was also familiar: just getting fucked. Plus breasts, swinging in time with Lydia's thrusts, which was super hot, so Stiles cupped them in his hands. Lydia was so beautiful like this, so fucking perfect, like she was made to do this.

"Of course I was," Lydia replied.

So, okay, he'd said that part out loud, that was fine. But there was a little too much going on right now for him to talk, what with Derek strong and warm behind him, holding him open, and Lydia fucking him, hot and intense and thorough, and Erica was right there behind her, a hand on Lydia's shoulder to keep her steady. He didn't want it to end but he also really wanted to come, so he reluctantly took one hand off Lydia's breast and reached for his cock.

"Let me do that," Erica said, which was also awesome because then they'd all be touching him, all helping him get off. Plus Erica would always reach between her legs to get her hand wet so she was like, jacking you off with her own come, and that was hot as fuck.

Hot enough that he wasn't going to last much longer. When he felt the orgasm coming he focused on not squishing Lydia's breasts, though he'd been teased enough that he didn't have that much strength left, and just flopped backwards, breathless.

Lydia pulled out and took off the harness, a little of Stiles's come on her stomach. He was glad that Derek had insisted on cleanliness, because he didn't want Lydia to be grossed out at all. He wanted her to do this again and again and again, to do it to Derek, too, while he watched. Then he thought of something.

"Did you come?" he asked her.

"It felt good, I liked it, and orgasms aren't everything," she said.

"Bullshit," he replied. "I didn't make you come."

"It's not the same and you know it."

"Lydia," he said, because she knew, she knew how he felt about making sure everyone came. He always kept track, especially when all four of them were together. It was important to him.

"Derek hasn't come yet," she said.

"Derek is going to do what he always does when he spends an hour rimming me and making me crazy and then gets me off: he's going to pick me up and fuck me like I'm a rag doll, and it's going to be super hot and I'm going to wish that I could come again," Stiles said. "Or maybe I will; it just hasn't happened yet. Now back to you."

"I got close, I just—it didn't get me off," she said shrugging. "Now I feel overstimulated."

Stiles felt like all the air left his body. "Really?" he asked.

"What?" Lydia asked.

"Yeah, so," Erica said, "as the person who watches porn with Stiles? I should tell you that he's pretty into forced orgasms."

"Not that I would ever force you," Stiles said. "Any of you."

Erica nodded. "And Lydia, I know from when we practiced with the strap-on this week that you got fidgety and weird after we fucked and I ended up going down on you anyway."

Lydia turned to her. "Tonight is supposed to be about Stiles."

"And Stiles wants you to come. Fake protests a bonus."

"Fake being forced?" She cocked her head. "Well, if you put it that way. Two birds, right?"

"Yes!" Stiles said.

Erica flipped off the bed and grabbed the big vibrator out of the nightstand drawer.

"Now that that's settled," Derek said, and pulled Stiles up, lifting him at the hips and thrusting into him easily, because he was so loose and lubed up.

Stiles let his body fall back against Derek, wrapped his arms around Derek's neck and settled in for a good pounding. "Fucking love this," he said. "Even better after Lydia."

"Yeah," Derek said, breathless.

"We should do this every time she fucks me."

"What makes you think I'm going to fuck you again?" Lydia asked, as Erica switched on the vibrator and knelt behind her.

"You fucking loved it," Stiles said.

Erica was whispering in Lydia's ear, putting the vibrator right on her clit without much warning, and Lydia flinched away from it. Somewhere in his brain Stiles knew that in those whispers Erica was coaching Lydia on how the girls in the forced orgasm videos acted, but that was fine with him, that they were faking this for him. Better, actually.

"No," Lydia protested.

"But Stiles wants you to come, so you're going to come," Erica said, holding her tighter, chin over her shoulder.

Lydia moaned through gritted teeth.

"See? Your body wants to come. Just let it happen."

Lydia slapped at Erica ineffectively, but her shoulders were pushing back into Erica as her hips thrust into the vibrator.

Stiles was feeling pushed around, too, manhandled by Derek into just the right place, and so fucking turned on even though he was only about half hard. Maybe Lydia had a point about orgasms, but it was his birthday and he wanted to make everyone come, directly or indirectly.

Lydia opened her eyes and looked at Stiles, who thought he probably made a pretty picture all fucked out and draped over Derek. He smiled at her and he could just see her relax, and then she was coming.

Derek, too, by the way he was pulling Stiles against him, hard. Maybe Lydia could also make people come with her eyes. They'd have to try that out.

Stiles sensed rather than saw Erica get off the bed, probably to get the toys into the sink, and then she was back with some wet cloths to keep them all from getting too sticky. Then she joined them on the bed, which was just about big enough for all of them if they arranged themselves right, a little overlapping, but that was kind of what they wanted.

"So, was it a good birthday?" Erica asked.

"Fuck yeah," Stiles said. "But it's not over yet. Not until Lydia fucks Derek with that gorgeous strap-on."

"I can do that," Lydia said. "If he's willing."

"Willing," Derek said, snuggling against Stiles. "Give me twenty minutes and I'll also be able."

"Good suggestion," Erica said, grinning. "Because that's going to be hot as hell."

"You know it," Stiles said.

"How are we going to top this for Lydia?" Derek asked.

Lydia laughed, low and a little evil. "Oh, I have some ideas," she said.