Derek is covered in a thin sheen of sweat, pupils dilated, panting like he can't get enough air. Stiles touches his cheek. "You're burning up," he says. "We should get you to Deaton. Is this a wolfsbane thing?"
"No." Derek leans into Stiles' touch like he wants more, but then pulls away. "It's just—" He lets out a heavy, anguished sigh. "I'm in heat."
Stiles stares as Derek lowers himself gingerly to sit on the end of his bed. "Heat," Stiles echoes. "Heat. Like, 'the dog is in heat', heat?"
There's accusation in Derek's weak glare, but desperation, too. "Yes, Stiles. The dog is in heat. I'm not sick, you can go."
It takes a few moments for Stiles to parse it before he moves. He heads for the bed, drops down beside Derek. "What do you need?"
Derek lifts his head like he's exhausted, surprise, even hope in his expression. Then it's gone. "I don't expect—"
"Stop," Stiles says.
Derek stares, eyes glassy, breathing hard. Then he presses into Stiles' side, like he can't get close enough.
"I'm here," Stiles whispers. "Tell me what you need."
Stiles has seen Derek vulnerable before. None of that compares to the moment Derek's resolve breaks, when he closes his eyes, twists his fingers into Stiles' shirt and tucks his face into the curve of Stiles' throat. "I need you," he moans, and Stiles' heart breaks for him.
"It is a sex thing, right?" Stiles asks.
"D'you wanna fuck me?"
Derek whimpers and shakes his head. "I need you inside me."
Stiles' brain seizes.
"Please," Derek moans, mouthing at Stiles' jaw and pulling at his clothes. "Stiles."
"Okay." Stiles tugs his shirt over his head, then holds Derek's face in his hands and presses their lips together. "Yeah, I can do that."
Stiles imagines it like the handful of times they've done it before, but with Derek on his back instead of Stiles. But when Derek gets out of his clothes he crawls into the middle of the bed and presses his shoulders into the mattress, thighs spread, ass in the air. His hole is pink and puffy, glistening, like he's already been fucked. "Did you—" Stiles wonders, dragging a finger around Derek's rim.
Derek shakes his head, quivering under Stiles' touch. "Please."
Stiles brings his finger to his mouth. The taste is reminiscent of precome, softer somehow, sweeter. It's slick like lube, and two fingers slide easily into Derek's body.
Stiles lets out a moan. "So hot." He imagines what it'll feel like around his dick. He's going to come as soon as he gets inside.
Derek shudders. He pushes back. "Fuck me, Stiles," he begs, twisting, reaching back to grip Stiles' thigh. "Need it."
"Yeah," Stiles murmurs, sliding his fingers out of Derek's ass, wrapping them around his dick. He's achingly hard, leaking precome that's slicked down his shaft. He presses the head of his cock to Derek's hole, pushes it in with his thumb.
"Holy crap." Derek's ass seems to grab him, dragging him in. He pushes forward involuntarily, sinks deep in one accidental thrust. Derek makes a deep, resonant sound of satisfaction.
They still for a moment, Derek sighing as he clenches down. Then, "Move," he says, his voice a rasping groan.
Stiles does, pushing upright, holding Derek's hip with one hand, stroking Derek's cock with the other. He pulls back, slides back in. His orgasm teases at the edge of his consciousness, threatening to break at any moment, and Derek's soft grunts intersperse with cries that grow higher in pitch with each thrust.
Derek stiffens, ass clenching down on Stiles' dick. Stiles gives another thrust, another stroke, and Derek starts to come with a long, drawn out groan, his body jerking with every spasm.
Unable to hold back, Stiles pushes deep and hard, shuddering with each pulse as he empties his balls into Derek's body.
They collapse in a sweaty tangle of limbs to catch their breath.
"Heat," Stiles says. "I didn't know."
Derek sighs, closes his eyes. "Born werewolves. Once every few years if you're lucky. But now I'm with you." He opens them again, and there's a crease of worry between his brows.
Derek nods. "I can stay away if you'd rather not— A few days, and it passes."
"Hell no," Stiles says. "I'm going to look after you."
"For a change?" Derek's lips tug up at the corner.
"Ha," Stiles scoffs, and drags Derek into a kiss.