I think it depends a lot on each person's view on the concept of ntr in the game, for me, my person, ntr is when you're in a serious relationship with someone and they betray you, and the only person I really see if Fitting into this is Alexis, both because she has already cheated on Mike, and because she continues to cheat, and the only thing you can really do is not invest in her or choose all the options that block this patch of hers. Apart from Alexis, I don't think there is any other character that fits this, all the other girls are either not in a relationship with you (like Samantha), or they suffer sexual abuse (like Shiori and Aletta) or they only have sex with another guy if you want (like Bree, Sasha and Lexi).
Even harem and lesbian sex only happen if the player commits to keeping the girls' status at the correct percentage, otherwise they end the relationship themselves.