Jim had plenty of experience, thank you very much, but he still got rather excited, especially in a situation like this, where they were doing it somewhere they really shouldn't. That and, well, the floor is not exactly as comfortable as a bed, and he has lots of experience in beds. Also, it's Spock, and Jim has never had a lover quite like him before.

And yes, feeling Spock's quite hard erection had only spurned him on that much more. He was pretty fascinated by Vulcan anatomy after it had been explained, and then showed to him. After all, he'd had various alien lovers before, but never a Vulcan, and certainly not a Vulcan male. He was actually pretty envious at first, but the sad thing about Kirk was that he was prone to becoming more easily aroused than Spock was, and his penis wasn't likely to be sheathed on a permanent daily basis.

Once they're both blissfully naked, he pulls Spock back down to him, and groans against his lips when there's all skin on glorious skin. Their erections brush, and that just sends him rutting and grinding his hips against Spock's that much more. It's all hot heat, sweaty, but already made slick from the Vulcan's natural lubrication, something that always made spontaneous sex quite... fun.

"I'm really not gonna last long at this rate," he huffed between pants.