Kylo is extremely well endowed, and rather than being all exited and turned on by that, Hux's reaction upon discovering it for the first time right before they're about to have sex is more along the lines of "...Nope. Nuh uh. That is not going inside me, I like my internal organs where they are thank you very much."

How they work things out from there is up to you, as is how much of a relationship they actually had before this incident, though I would request that there be no non-con involved.