The NTR so to speak is split into two different categories. One is called "Public Use" and it is a form of mind control that you can do to your girls in order to have them have sex with other men for the purpose of corrupting the masses. This isn't strictly speaking NTR as they are just doing your bidding. You start this by unlocking their final level of corruption and activating the option. However, you do not brainwash your girlfriend Emma and you can start a Netori path for her if you A. Let her help with corrupting Luca and B. have a discussion with her afterward that you like seeing her with other men. Emma will not have sex with other men if it isn't arousing to you, but once she is given the green light she will take every opportunity that comes along whether you are watching or not. As of yet, there isn't any NTR that the MC doesn't at least implicitly want to happen. No real humiliation or loss of love. Emma can talk some shit, though, even though she is only doing it because she knows MC likes it.