Hux and Poe had a one-night stand ages ago, before they knew who the other was. Sure, it turns out that they're mortal enemies, but the sex was smoking hot. If they have leave at the same time they'll meet on neutral planets and have filthy weekends together. They have burner phones comms purely for sexting. They can't actually hold a conversation, but that's not really what they want, so who cares? When Poe is captured Hux has to delegate the interrogation to Ren, because if he does it himself it'll turn into a ~sexy interrogation.

Bonus: Hux's main squeeze is Kylo- Poe is his bit on the side.
Double bonus: Poe is not sure that he wants to be exclusive with Finn if it means giving up Hux. "I really want to keep having phenomenal, depraved, meaningless sex with your evil ex-boss," just doesn't sound great.