The apocalypse is hard to deal with, and it turns Martin cold. Jon is desperately in love with him and doesn't really have a frame of reference for a healthy relationship, or maybe he just feels like he deserves it all.

Maybe Martin takes his apocalypse anxieties out on Jon physically. Maybe he weaponizes Jon's self-loathing, guilting Jon into do things that hurt him. Maybe he demands more physical affection than Jon is comfortable with. However you spin it, Martin consistently hurts Jon and violates his boundaries. Explicit sexual coercion/dubious consent is welcome, but not necessary.

This would be lovely as just a slice of their terrible relationship, but I would also love if you wrote Jon and Martin encountering someone else - Georgie and Melanie? - who immediately notice that something's up and try to put a stop to it.