The reason why you like Amicus in the beginning is because you felt special and appreciated. This is more of a personal thing, but I see sex as extremely emotional. Meanwhile, it seems to be a common theme that the wolves see sex as purely instinctual. When you see Amicus with Neferu, if you were like me, yeah, you can forgive him (which I do) but I can't convince myself that it wasn't emotional because that would be going against my personal meaning of sex. It is clear that that's not Amicus's priority especially when he's trying to get a handy from you in the bedroom or a blow in the prison.