Just giving a comment about the NTR talk... for now all the girls we have met are really "pure" in the sense that they seek someone to love and in fact in many cases they left the demon city out of the corruption it has, many monstergirls that we met tend to talk about the demon city in a bad perspective and in fact in this place is where I feel that things can go wrong, like a Kotone without remorse for example, so if it ever gets added it will be there.
About if I think if it should be added anywhere else... I would say no, perhaps some open relationships, threesomes with you and two girls, at most if the game ever gets a futanari girl a threesome of that type. As some other people said quite a few relationships start on an "open" stage then gets really close (such as our catgirl in debt) and that's fine.
Personally I kind of hope to see level draining and more permanent debuffs that you can get in certain situations (even if later you can find a way to get rid of them). That type of stuff is what I find fun, having enemies becoming stronger and you becoming weaker with related consequences.