Yeah, I don't know how so many don't understand this. To expand on your post - NTR is the MC's love interest cheating on them regardless of gender. For example, if the MC is a woman with a girlfriend/wife/partner and that LI cheats on her, it's NTR.
"Cheating" (netorare) is NTR. Written purposely to cause jealousy in the person reading the story.
Group sex, swinging, sharing with MC's consent is not NTR. Just because an individual reader might not like swinging, and seeing it in a story makes them feel jealous, still doen't make it NTR.