Pre-TFA: Those who have met General Hux in person knew he was gay, maybe not right off the bat but he made no secret of it. And those who didn't at least heard about it. Everyone except Kylo Ren.
So when the General starts to flirt with him, Kylo is utterly confused at first until he chances upon a conversation between crew members about the General's obvious campaign to win the knight's heart ... or his body.