You don't have to be abusive, I mean, not in a brutal way. When you succeed in dominant choices, the girls "consent" to your actions and often lose willpower. Ofc if you don't want to have sex with them at all, it gets harder, but you can always "explain" things to them, once per day. Some of the interactions have two willpower hit events, once if you succeed, then another if your next reply (careful or dominant) also succeeds, both ways give you a willpower hit, the key is to succeed in the interaction. I think it's at least a 3 or 4 willpower hit if you always succeed, maybe more, even without using threats. Being dominant in the bathroom also may hit their willpower. With lectures, I wouldn't be surprised if you could cut off at least 10 willpower per day without sex. Ofc, if you add sex, you can wipe out a lot more. No need to rape them, be dominant but give them enough foreplay that you don't keep failing.
It's a lot worse trying to raise willpower. All I could ever assuredly get was 5wp per day, 3 from lectures, 2 from praising intelligence. Makes you re-roll a lot if you want to power game.