Both of them have mommy issues. Hux never really knew his and was raised by a droid while Kylo is desperate for any kind of affection, but he especially misses Leia's. Plus, you know, incest.

They end up roleplaying with Hux pretending to nurse Kylo and "mommy" cooing sweet words to her boy while he suckles and gets spoiled with attention.

How they fuck is up to A!A, but I'm always down for creampie!Kylo.