I had no clue what NTR was before i came here. Its hillarious how upset people get over it.
If you read the posts here, then you have the wrong impression. It literally means that a loved one is taken away from the main character and the main character must catch them. Sharing willingly or swinging is not ntr. The main character must be against it and suffer, it's a japanese drama mechanic. It's not so much about sex than it is about the loss (making out would also count, on stage actors would not bang each others ... most of the time).
Many people however think it means that we see someone else than the MC banging one of the girls, no matter if she is his love interest or not.
And yes, some get a kick out of it and some hate it to the bones. Those two factions fight each other to the teeth.
I personally don't mind much. If it's part of the story, why not. But only putting it in so people see someone else having fun ... nah.