Kylo and Hux don't get along in their professional lives. They just really, really don't. They don't respect each others' abilities and they're constantly vying for Snoke's attention at the expense of the other.

They both think they're crazy when they decide to have sex for the first time and are convinced it'll be terrible. Except it turns out that even though they're at odds in running the Finalizer, they're incredibly compatible in bed. Kylo being the emotion and passion to Hux's control and finesse works perfectly in the sheets. Basically, it's the best sex they never knew they needed.