What is this guy talking about then??
It sounds to me like they are referring to the mom (MC's
former wife), who had cheated
prior to the game beginning. That's not NTR in the game, because it's already in the past. Whether you forgive her or not is up to you, but in the game itself there's no NTR at all.
Also, the developer updates here themselves and there's no tag. If there were actually NTR (regardless of what someone claims), this would have the tag via a mod or staff if it were tagged incorrectly. To clarify, the choices (beyond forgiving the wife or not) in the game tend to be more things like making the niece a calm, measured superheroine or having her unleash her dark side on everyone. There is no, "Hey, that dude [or girl] over there looks super cute, so let's go get him [her]!"
It's possible, I guess, that if you've rejected a potential LI that she's with someone else (I never did), but that wouldn't be NTR either, because she's not a lover of the MC at that point. And again, the wife issue is from before the game even begins (the MC is alone except for his daughter and after a little his niece).