Written for this prompt on trekkink:

"Spock's Pon Farr is coming up. He and Kirk have discussed it and Kirk is, of course, going to help Spock through it.

Thing is: Kirk hasn't ever bottomed and he's never been interested in bottoming. He's not going to let that get in the way of helping Spock, though. He fingers himself to get ready and make sure he can jump right in when Spock goes into Pon Farr. That doesn't prepare him for how great it feels when Spock fucks him for the first time.

Basically: Kirk discovers he's a total cockslut for Spock during Spock's first Pon Farr."

That being said: enjoy!

No matter what anyone in Iowa, or at the Academy, or even on the Enterprise said, James Tiberius Kirk was no bottom. He think it's the lean body and the boyish looks that give off a distinct "I love cock" air, but Jim's never bottomed in his life. Of course, he's never had sex with someone he loved, either. But Jim is crazy in love with Spock, and he knows Pon Farr means that Spock's going to have to jam his dick into the closest, tightest thing he can find. And Jim's going to make sure it's him.

Spock says he thinks they should get started after shift, seeing as he's sweating and he thinks, illogically, that his blood might be boiling and his skin is surely burning. Jim leaves the bridge early to get ready. The most he's had up his ass is some weird, vibrating dildo once, so he's a little nervous. He takes the apple flavored lube (because fuck, he loves apples) out of the drawer of his bedside table and applies it to his index and middle fingers. Here goes nothing, he thinks before he slides his first finger in up to the knuckle.

It's weird. Not entirely bad or anything, just...different. He pulls it out until his nail is scraping the skin, then pushes it back in, farther this time until his whole finger is inside him. He wiggles and twists it, his body knowing instinctively that he needs more. With a shuddering breath he pulls out before adding his middle finger. He pushes them apart to make a V, opening himself up wider. He adds more lube and crams three fingers inside, falling forward onto his hands and knees as his body trembles and bucks back against his hand. He brushes his prostate for the first time, colored lights popping behind his eyelids, when Spock walks in.

Jim doesn't even hear the doors open, but Spock's feral growl alerts Jim of his presence. "That should be me, Jim," he snarls, slapping Jim's wrist, making his hand fall away. "And it will be."

"Please, please," Jim hears himself beg, quiet and whiny. Spock rips his blue sweater and undershirt off, and Jim does the same, hands shaking against the gold threads. The black pants are next to go, Jim's already around his knees.

Spock kneels behind Jim on the bed, losing some of his almost perfect control as hot hands grip Jim at the waist and wet lips suck on his spine, drinking his captain in. "Where is it?" he asks roughly, and Jim needs no clarification.

"On the bed to your right."

Spock blindly grabs the bottle and opens the top. "Apple," he says, and Jim's surprised Spock still has the capacity to be amused. Jim can smell the lube, the fresh, fruity scent almost too much for his heightened senses, as Spock coats himself sloppily. "Jim, I cannot-," he tries, but doesn't finish. But Jim knows what he means: "Jim, I cannot take the time to touch you and learn every part of your body as I normally would. I cannot be slow and gentle. I can barely even think."

Jim gives a hasty nod, and then Spock is driving into him, one hand around Jim's waist and the other gripping the sheets on Jim's bed. "Oh!" Jim cries out, pushing back, and this is so much better than his own fingers.

"Jim," Spock chokes, a strangled sound like that of an injured animal, and then he's snapping his hips again and again, and Jim can barely keep his eyes open. "Jim, oh, you feel..." Spock trails off, too lost in sensation to continue being coherent.

Jim, despite his earlier protestations and reservations, is loving every brush of Spock's dick against his prostate and every slide of Spock's skin against his own. "Yes, Spock, more, more!" he cries out, jaw clenched and teeth bared.

Spock picks up his head and moves his mouth close to Jim's ear. "Are you enjoying yourself, Jim?" he asks.

"Oh, God, YES!" Jim cries. "More. Just...harder. Fuck!" His breath comes out in harsh stutters and his lungs burn, but the only thing he's aware of is the mattress creaking and Spock's body over his. "Deeper, Spock. Need you deeper."

Spock grunts his assent and moves so his knees are basically over Jim's thighs, and he feels like an animal but he doesn't care, can't care. "James," Spock says, his voice scratchy, and Jim shudders at the use of his full name. "I need you to do something for me."

"Oh, Jesus, anything," Jim moans, head bowed and neck tight.

"I need you to come all over the sheets for me." Jim moans, and suddenly Spock's hand is wrapped around his impossibly hard cock, and it only takes one quick tug before he's doing just as Spock asked, his whole body tight.

Spock nearly howls as he follows, fucking into Jim until his orgasm subsides. As soon as he regains his senses, he notices his hand is sticky, and proceeds to suck the offending substance from his palm and in between his fingers.

Jim turns his head and groans. "Fuck, Spock, if I didn't just shoot my brains out my dick I'd be hard right now."

Spock huffs, and Jim recognizes the noise as a laugh. "So is it safe to say you enjoyed 'bottoming'?"

Jim slumps down, not caring that his stomach his now messy. "Fuck yeah. When can we do it again?"

"Well..." Spock responds, looking down at his cock, hard and leaking. "Now, if you wish."

Jim flips over so he's sprawled on his back and smiles. "Yes," he hisses, and Spock fills him again.