It would be nice to have a game, for once, with "proper" NTR in it. I am not sure this is still possible though since we have already given the wife a chance to cheat.
What I mean by proper NTR, is set up a circumstance where the loving wife/girlfriend is "forced" to cheat on the MC. It is not her choice and not something she would do willingly. The MC is aware of it, and though he disapproves and is jealous and upset, he watches her do it. Despite herself, she becomes turned on and enjoys it, but doesn't initially admit it. This happens multiple times, and every time she enjoys it a little more until eventually she is an unrepentant slut doing it willingly, and even seeking it out, while humiliating the MC.
I prefer a non-violent and even non-coercive "forcing" personally, where some extenal factor makes them do it. (For example, maybe the MC is in a life threatening situation and to get something she needs to help him, she has to fuck someone.) But simple blackmail, or paying off a debt, are probably easiest. I remember one, admitedly far fetched, story I read where the wife developed a medical condition that could only be cured by drinking cum directly from the source, and of course the hudband's source turns out to be incompatible...