Best cheap trick
Saying she was not in relationship with mc so it's not ntr
NTR is technically an acronym for three distinct fetishes: netorare, netorase, and netori.
Netorare, the one people almost always shorten to NTR, loosely translates to something like "to steal someone away by sleeping with them." The most obvious example of this is some dude banging someone's girlfriend well enough that she dumps the boyfriend because she's addicted to the other guy's cock now. It's become increasingly common for people to put the NTR tag on works where the MC has the hots for a chick, but she hooks up with another guy--which isn't actually netorare, it's just the MC being rejected by a girl who may or may not know how the MC feels, and may or may not care. Sometimes people apply the tag to works where a family member is "stolen" without having ever been in a relationship with the MC; the justification there is that the family member no longer gives a shit about the MC, so they were "stolen" from them in that sense.
Most of the time Netorare isn't really about anyone being "stolen," so much as realizing that the love interest
belongs to the streets. Such works are less about someone making the MC seem inadequate as a male than it is ripping away the illusion that women value love more than pleasure, chivalry over masculinity, or soy over testosterone.
Netorase is regular cuckoldry. The husband or boyfriend
wants to watch someone plow their girl. Even if their woman fucks around behind their backs, if they're happy about it when they find out it's still Netorase if they're still "together."
Netori is basically what the "Cheating" tag used to be; the MC is the one doing the stealing. It was very common for the women being stolen to be lonely housewives or have dirtbag boyfriends, but there were rare works where the other boyfriend was just a loser and the MC decided to make a move because he could.
Anyways this game is not for me
Vomits and walk away
This is the mature thing to do. If it's not your fetish, it's not your fetish.
People like to cry NTR when artists and developers acknowledge that modern women are very rarely chaste, and usually start having sex in their teens--it's not NTR, it's just a truth that makes people uncomfortable.