The NTR or at least NTR-ish stuff came out of the blue for most of us. Yes, the mother had been in a relationship with that guy before. But that was in the past. We thought he was long gone. Then apparently he shows up and bangs the mom right away, with no way to avoid it. I heard about it ahead of time, so just didn't play the update, since that was the only way to avoid.
I don't mind if a game has avoidable NTR content. It can have pretty much whatever controversial content it wants, as long as it's avoidable. If the game is outright an NTR game, that rule doesn't apply of course. But this game was never billed as an NTR game. In fact, it seemed fairly safe from NTR given the circumstances. Now, if the boyfriend/father/whatever had been introduced, and he's fighting you for the girls, I'm fine if him getting them is the penalty for you failing, as long as it's not an impossible task. There's something to be said for the thrill of "I need to save her from that predatory asshole because I'm the only predatory asshole allowed to touch her." I would view the other guy getting the girl as a "bad ending" whether the game actually ended or not, and then go back to a previous save and try harder so it never happens.