Note: Yeah, this is really short, but I had this idea while I was taking a shower (weird, huh?) and it just needed to be written. The plot bunnies: THEY HAVE BITTEN. And I have listened.

Rating: M

FYI: I don't own Star Trek or anything associated, so don't sue, kthnx.

It wasn't the first time Spock had cheated on Kirk with another person before. Hell, it's not as if Kirk even really cared; what they had was… special. It wasn't a relationship, but it wasn't just casual fucking, either. What they had was theirs and they were okay with that.

But it was who Spock cheated on him with that had his mind reeling. So when they were having sex that night – it was Kirk's turn to top – he questioned Spock about it.

"Why her, Spock?" he asked, thrusting deep into the tight heat that was Spock's backside. "I mean, you could've picked anyone else, but why her?"

Spock looked at the Captain over his shoulder and raised an eyebrow as if to say, "Are we seriously talking about this right now?" He opened his mouth to speak, was cut off as Kirk dug his fingers into his hips quite painfully, and then continued. "Nurse Chapel is quite aesthetically pleasing, as well as quite intelligent. I enjoyed her company."

"No," Kirk replied, starting to thrust a bit harder now that his anger was rising. "You just wanted to fuck her brains out."

"I do not see anything wrong with that illogical desire to do so, and I have already stated my reasons for feeling that way."

"Yeah, but… you cheated on me."

"It has never seemed to bother you before." He had to grab onto the headrest to stop from sliding into the wall, as Kirk's thrusts had become unusually rough.

"But you promised you would never sleep with her."

"I lied."

Kirk bit his lip, moaning. "I thought Vulcans never lied."

"…It was necessary."

"Well, I'm making you promise now: no more cheating on me. With anyone. Understood?"

Spock moaned on one particular thrust, pushing himself back into his Captain's hips. "Understood, Captain."

Kirk came, then, buried deep inside his First Officer as he rode out his orgasm. He collapsed onto his back, bringing Spock with him, and snaked his hands down the Vulcan's stomach to his erection, which he began to stroke earnestly. "I want you all to myself from now on," he whispered, placing a kiss behind a pointed ear; he felt Spock's hair move against his skin as he nodded.

"Yes, Jim. I am all yours."

Kirk moaned, squeezed, and that was all it took for Spock to release all over his own chest.

They lay like that for a few minutes before Spock said, "Captain, if you would release me, I wish to make myself comfortable and decent."

Kirk let go of him, watching as his First Officer went into the bathroom, returned with a wet cloth, and cleaned them both; he sighed in contentment when Spock settled into the sheets next to him and wrapped an arm around the broad shoulders. "Thank you, Spock."

"Your gratitude is unnecessary; I just wish for your happiness."

"I'll be happy when you stop cheating on me for good."

Spock looked over at him seriously. "It is done."

Kirk smiled and relaxed, falling asleep in a short period of time, Spock right behind him.

Listening to: Muscle Museum - Muse