Han used to be in a sexual relationship with Luke. Now, Kylo (or Ben, if you prefer- I'm very flexible on the scenario) is fucking Luke, and gets irrationally competitive- always fixating on "did Han fuck you this well" or "did Han make you come this hard".

Just Kylo/Ben being determined to outdo dad.

Scenario, timeline, how Luke feels about this, if it's con or not- all up to the filler.