Because there are a lot of sensitive human beings.
You have to look at it in a Visual Novel perspective not in a Porn/Hentai perspective.
NTR/Cheating is despised on and should never be normalized however there are people who even get triggered by the slightest chance of NTR happening.
We still have to understand that it is a kink and how i dislike BDSM and domination, which is in almost every VN respectfully and forced, i don't let it bother me. Sadly so, NTR seems to be a soft-spot on many people's minds.