Either Kylo hasn't had sex before or he hasn't had any in a long time. He's repressed and overly sensitive and maybe a little bit desperate. So the first time he gets with Hux, he finishes quickly. REALLY quickly. Possibly in his pants.

He's incredibly embarrassed while Hux is incredibly amused. It can end there or it can go on with Hux taking it slow with Kylo next time and drawing the sex out longer. Up to you!