They strike up a relationship of sorts. Obsessive and and mercurial and a little possessive, and the sex just about blows kylo's mind. Like...wow okay, he's never been fucked so good his thighs shake but okaaaay. Wow.

Then he gets attached. And he can't want that. It pretty obviously isn't what hux would want, and at the beginning that was fine but now he almost dreads when they sleep together because it's just a reminder that he gets this, but no more.

He's not expecting hux to notice (why would he? even if he did, he wouldn't care.)

Hux does.