Kylo, for all his attempts to be stoic and in control, really isn't. The same goes for when he's having sex. He tries to be that person who just grunts every now and then but naturally he's super responsive and super vocal when it comes to pleasure.

Hux has a field day with that.

Bonus (but not necessary at all!): They have sex where other people could presumably hear them if one of them gets too loud.