The so called NTR that people are losing their mind is the milf character, possible future love interest for main character, banging her husband. Yes, it is completely avoidable if you choose so. Yet seeing the scenes will actually explain her characters lore much better, as you will find out that she is an exhibitionist. This obviously is the branch that quite possibly can lead the main character getting to do stuff with her, as her husband isn't too keen on with doing the exhibition stuff, since he is so conservative. So she might have to find someone else to help her out with her fantasies, and since the only other male character in the game just happens to be the main character, it ain't too difficult to guess that he will go for it.