Well, on the most basic level, he was a living and breathing man. How did one not when presented with the opportunity such as this not have a very specific arousal? It was also a very nice happenstance that it was Kitty who provided said bits of excitement.
St. John wasn't a week man in any light. Sure he wasn't strong like Blob, or some of those other X-Men, but he kept himself in shape and ready to go. Then honestly, it wasn't like Kitty weighed that much to start with. But the question was how long he could last in this. A smirk spread across John's lips as Kitty teased the tip of his nose with her tongue. Light sexy times were the best. Sex was supposed to be fun, and this was fun.
"Somethings are just too exciting to be patient for." John moaned as his hips moved very pointedly to drive himself up into her. The way her moan sounded. The way her movement matched his to have him. It all was intoxicating, and John was all about making her make more of those noises. Especially after she complimented him so.
John's strong hand pulled up on her thighs more so he could more freely thrust up into her trying to bury everything he had to offer into her. Mouth on the front of her neck sucking lightly to encourage more delightful noises out of her. "Kitty," He moaned grinding his hips into her. Something about being in the hallway with her, and her specifically made this all the more delightful.