What's Netori?
What's taboo about this?
Cuckolding someone. The protagonist make a love interest who is already in a relationship with a third party into a physical relationship, and the love interest comes to like it over time, thus either cheating or breaking up with their original partner.
There is not and wont be any violence of forced sex relationships.
Taboo an incestuous relationship, They are two separate topics and have nothing to do with each other.