The Dev intended for the story to portray the MC mom in that way, a "seemingly" hateful person who made his dad miserable, and that NTR scene reinforces that concept, to the point of putting the player on a fork choice between trying to understand mom or working his way towards blackmailing/revenge against her.
NTR works for those who do not mind it, though I can understand how some people can't get over it, thus, even if it means showing less scenes, I'd also suggest the DEV to give an option to hide the NTR content in the game, which, while it will still happen canonically, it won't be shown to those who chose so.