In the course of one of their groping UST slapfights, Kylo accidentally finds out Hux is ticklish. So he does the sensible thing: straddles Hux's stomach, holds him down with the Force, and tickles him mercilessly. It should be a simple exercise in humiliation, Hux squirming and helpless, but Kylo ends up hard, and eventually jerks off on Hux's belly. Hux is weirded out but decides to roll with it (Kylo's kinks could be worse). But Kylo's so embarrassed after he comes on Hux that he just gets up and flees the scene. Leaving an indignant Hux on the ground covered with jizz.

++ it's not even the weirdest sexual encounter Hux has ever had
+ Kylo was not, in fact, aroused specifically by the tickling and is even more embarrassed because hOW do you live that down??
+ Kylo loses all ability to loom menacingly around Hux forever