Chapter Text
It's getting late. The daylight cycle shifted to evening a few hours ago. The sun has set, leaving behind light from an artificial moon to shine through panes of xenonite. Soft shadows cast around the room, objects illuminated by the bedside lamp. The bed frame thuds against the wall with rhythmic motion. Thankfully, they're reinforced with lattices of xenonite. Nothing it can't withstand.
"Hah—oh fuck—please," Grace whimpers at a particularly hard thrust. He paws at the blanket beneath him, gathering the material tightly into his fist. His pliant frame is smushed into the mattress, weighed down by Rocky's strong arms. Grace leaks profusely: tears, drool, pre-cum. It's quite a mess, but he couldn't care less. He indulges in the sensation of being split open by Rocky's unforgiving girth. He just wants to take it, more and more.
That is, until Rocky's movements cease. Grace can't feel anything except for Rocky's carapace flush against his ass and the bruising grip on his hips. No, no! He wants more. He wishes Rocky would take him again and again—shove his ovipositor deep inside and wring out several more orgasms. But that wasn't their agreement, now was it?
"Continue your explanation," Rocky demands. Stone-cold. Without an ounce of warmth in his tone. Grace can't help but whine as he attempts to push back on Rocky. He yearns for some semblance of relief. Grace tries to rut his cock against the mattress, but Rocky responds by increasing the strength of his grasp. Honestly, to the point it hurts, but that's no issue. Rocky knows Grace loves it when he's rough. He loves when marks are left behind, showing proof of who he belongs to. Those delicate hues of purple, red, and blue. If only Rocky could see how beautiful he looked with his marks.
There are certainly a number of those marks forming on Grace's body now. But that is beside the point. Grace whines again, a long, nasally sound drawn from his throat. He knows what he's supposed to do. Rocky chirps the approximation of a tut, amused by the pathetically desperate display coming from his human. "Continue, or I will not allow you to come," Rocky repeats.
Oh, lord. Grace frantically tries to remember where he left off with his explanation of the citric acid cycle. Was it the step between Succinyl-CoA converting to Succinate, or maybe it was Succinate to Fumarate? Gah! Why choose Sir Krebs' cycle as a topic? Well, he wanted something familiar. Something easy to explain, in theory.
In practice, it's really hard to concentrate on describing each mechanistic step while getting your back blown by your mate. Whatever, he just needs to start back up somewhere so he can actually get off. "Okay, um- the step where Succinyl-CoA converts to Succinate. It ah- requires an enzyme known as Succinyl-CoA Synthetase."
Rocky draws back his ovipositor before ramming it back into Grace, forcing him to wail in pleasure. "Keep going," Rocky commands.
Grace groans, trying his best to maintain composure. "A phosphate group c-comes in and replaces the CoA—mnnnhh—which is transferred to ADP to make ATP. Depending on the cell type, GDP can also be utilized f-for the reaction, yielding GTP as the product. Oh!"
"Hmmm, I know ATP is adenosine triphosphate. What is GTP? I don't think you mentioned it yet," Rocky hums thoughtfully. But he pairs it with another sharp thrust, which makes Grace's toes curl.
"It stands for—aaahhn!—guanosine triphosphate. It's similar to ATP as an e-energy source. But uh- the usage of GTP is more common in tissues with g-greater numbers of anabolic pathways like—hhhghh—like in the liver."
"S'not hard to remember the name, j-just the first part changes." The last word is spoken with a hiss. Grace turns his face toward the bed in an attempt to mask his moans. His brain feels like putty, overwhelmed by pleasure signals zinging back and forth.
"What comes next?" Rocky presses. Oh god, he can't catch a break. Relentless.
As Grace begins to explain the next step, Rocky pulls him back by the hips. Each stroke—so very slow, methodical, and expertly drawing out whines from the human. It's not enough to quell the heat blooming through Grace's gut.
His voice is muffled by the fabric against his lips. It's not that incoherent, especially with how sensitive Rocky's auditory receptors are, but he relishes being a nuisance. "I can't hear you, Dr. Grace. Start over," he chirps out, amusedly.
Grace turns to the side and gasps for air. "Ah fffu- The conversion of Succinate to Fumarate involves o-oxidation. Two hydrogen atoms come in and- uh- transfer their electrons to FAD—mmnhh—yielding FADH2."
"What's the enzyme name?" Rocky asks with a deliberate tap-tap against his hipbone. It makes Grace twitch and shudder.
"Succinate Dehydrogenase," he mewls.
Rocky rewards him by increasing the pace. Just a little more to take the edge off, but still not enough. It doesn't last. He stills his carapace again as Grace stalls to provide information about the following step. "God—Rocky, please. I want—need more," Grace cries.
"Need what? Be specific." Tap-tap-tap.
"You! Please fuck me." Grace enunciates with a firm push back against Rocky's carapace, forcing his ovipositor to sink deeper. His poor, neglected cock weeps precum onto the sheets.
Rocky effortlessly flips him over onto his back, sending yet another wave of arousal coursing through his body. His cheeks are probably flushed embarrassingly red, but he tries not to think too hard about that. Instead, the absolutely erotic, looming presence of the Eridian standing above him, caging him like prey.
"Hmm, no. I still want to learn. Your explanations have gotten sparse," Rocky chides. Grace's heart thumps loudly in his chest. "Come on. Keep going."
Grace's speech strains. "Fumarate converts to Malate through the—a-ah!—addition of water. The r-reaction enzyme is Fumarase!"
Rocky resumes his thrusts, a little faster than before. He's doing whatever he can to rile up Grace. The stretch of his hole is mesmerizing, how it accommodates the girthy ovipositor, like he was made just to take Rocky.
"The last step of the cycle—hah—regenerates Oxaloacetate through the o-oxidation of Malate. During the p-process, NAD+ is reduced to NADH. The associated enzyme is—Aah! Mmngh!" Grace's explanation is cut short with a sudden flex of Rocky's ovipositor tip against his walls.
"What were you saying, Dr. Grace? I'm afraid I didn't quite hear that last part."
"Malate Dehydrogenase!" Grace whimpers while draping an arm across his eyes, attempting to hide his face. Tears stream down his cheeks. Such an alluring feature for Rocky to echolocate off of. He drums his fingers across Grace's chest, listening carefully to his hitching breath, his quivering muscles.
Rocky removes one of his hands, which had been planted firmly against Grace's hip. He trails his fingertips across the smooth plane of skin until it settles above the thatch of hair on his stomach. Grace writhes beneath Rocky, desperate for contact.
"P-please, I need..." he trails off, arching into the ghosting touch against his cock. Rocky's hand wraps around it and provides a languid stroke. Thank god, he stopped his endless teasing.
"Use your words, Dr. Grace. What do you need?" Rocky coos.
"I need you to fuck me, please." Grace replies, barely above a whisper.
Rocky hums approvingly. "Only because you beg so nicely."
Grace's eyes roll back as Rocky takes him. He's lost in ecstasy as Rocky drags his ovipositor in and out of his mate, paying special attention to aim the tip against Grace's prostate. Thank god for its prehensile ability. So good.
His attention shifts downward toward his cock—drooling precum against his stomach, bobbing with each snap of Rocky's carapace against his thighs. A little lower, and he can see the glint of Rocky's exosuit in the low light. Silver fluid stains the underside of his carapace. His ovipositor stretches the flexible ventral seam panel.
"Oh! Ah- oh fuuuck." Grace trembles helplessly beneath Rocky, feeling the hot buildup of pressure in his abdomen. Like a coiling band, it winds tighter and tighter. So close, so close.
He reaches for Rocky's arm, the one on his cock. Grasping something, anything, to stabilize the growing pressure of arousal. Each stroke of Rocky's hand times perfectly with his thrusts. Each brush against Grace's prostate brings him one step closer to—
"Rock—ohhh m'gonna come," Grace whines. It's no surprise to Rocky. He can hear the tremble of muscle as his mate approaches climax.
The hot band of pleasure snaps, washing over Grace's senses as he screams. He comes onto the underside of Rocky's exosuit. The rest lands on his stomach, joining the mess of pre and sweat. His exhausted body twitches as Rocky milks every last drop.
Rocky eases off Grace, releasing his spread legs so that they fall gently onto the bed. Grace's eyes flutter shut. Just a few moments; he needs to catch his breath. Grace feels his age with each passing day, but he can't help but relish the gratifying ache in his hips. Tomorrow, it'll be a pleasant reminder of how good his mate fucked him. And, as if Rocky were listening, he begins to massage the muscles along his glutes. Each knead of the flesh, so soothing, he groans in satisfaction.
Lingering thoughts push their way back to the forefront of Grace's mind. Now that his mind has reverted to a more rational state, he's feeling a little embarrassed. Explaining science while getting pounded to oblivion is not something he anticipated would come up in his sex life. With Rocky, anything is possible, though. He just hopes this doesn't interfere with his ability to lecture his students.
He's pulled out of his doze when Rocky stands to leave. Grace protests, "Mnnhh noooo, don't go." The arm that was wound around his carapace flops onto the bed unceremoniously. Rocky makes a bit of a ruckus in the other room. Grace hears the tap turn on briefly before Rocky's tip-tapping steps retreat to the bedroom. He's brought back a damp washcloth and a container of water. Rocky crosses the room and settles back onto the bed. He hums wordlessly, passing the water to his mate. Grace drinks it obediently.
When he's downed the water, Grace sets the container onto the nightstand. He winces as he attempts to sit up properly. Rocky helps, giving him something steady to lean on as he cleans up his body.
He folds the washcloth over to a clean side and wipes up Rocky's exosuit. "Uhg, gross. Sorry, it seems I've leaked all over your fine work of engineering."
"It's okay. I like it," Rocky chirps, giving Grace's torso a little squeeze of affection.
"Okay, you little freak," Grace replies lightheartedly.
"Do you wanna change out of your suit, or..." He gestures to the stains of fusion fluid coating the interior of his suit and ventral seam.
Rocky clicks his fingertips together, considering, before replying, "It's fine. The sensation is not noticeable. And, I can hear your internal functions slow in preparation for sleep. I don't want to leave you alone without anyone to watch."
Well, that's that.
Grace abandons the washcloth on the nightstand and turns so he can embrace Rocky with both arms. Rocky mirrors him and purrs, leaning into the touch. Love and absolute bliss.
Rocky announces in a mockingly formal tone. "I have learned much about the citric acid cycle. I appreciate receiving this private lecture from you, Dr. Grace."
Aaand, the moment is ruined.
Grace groans, "Oh, god. Please don't call it that. And don't call me Dr. Grace."
"But you like it." Rocky teases. "I can hear your heart rate increase by several beats every time I say it. Ooh, Dr. Grace. I feel it when you squeeze down on my—"
"Nope! Gonna stop you right there! I am not gonna hear you out." Rocky cackles, hooting numerous tones laden with humor. Grace attempts to push Rocky over, but he remains an immovable object.
Grace looks over at the clock, which reads [23:23]. Oh, it's late. He's got a real lecture to give to his Eridian students tomorrow. "I need to sleep," he grumbles.
"I'll watch," Rocky chirps.
"Yes, yes. Thank you, dear." His tone is sarcastic, but there is no real annoyance to it. Grace gives Rocky a quick peck on the top of his carapace, then rolls onto his back. He makes space for Rocky to nestle between the crook of his arm and his side. Rocky drones a soft and soothing vocalization, lulling Grace into a peaceful slumber.