NTR is also a much broader subject for those that dont like it than for those that do.
It's probably just as broad for those that do, unless you're insinuating that the "enjoyers" only enjoy the cuckoldery aspect and not the uselessly broader aspects of the category.
To me NTR is as useless a category as isekai, I mean when I can fit Star Trek S01E18 Arena† into isekai, it's a useless category. Likewise if "other person MC has feelings for continues to be in relationship with another person" is described as NTR it's a uselessly broad category.
i mean look at this way. you are good friends with this girl, you both start feeling more than friendship but neither of you have confessed but both know there is something there. and she then goes and sleeps with some other dude. would you not feel hurt and betrayed?
No, because I'm an adult*. I understand that projecting my feelings onto others
without discussing those feelings and having them reciprocated, doesn't mean they have the same feelings no matter how I misleadingly read the situation.
Real life isn't an AVN, you do not know what someone else is thinking or feeling
unless they tell you, and even then, don't be surprised if they do things you think are in contradiction with their
expressed feelings or thoughts. Because like, people change their minds or are sometimes just inaccurate and inept about the very messy subject of feelings. You aren't the arbiter on what is "the right way to do things", everyone stumbles through life "doing it their own way". Learn this as fast as you can, embrace it, embody it, and it will save you pain in the long run.
* When I was a teen, yes, I felt hurt when the person I was all but actually verbally expressing my interest in returned to a previous relationship with their prior romance. An abusive relationship at that. But, I got over it, moved on, and learned to verbally express my feelings because
people aren't mind readers no matter how much it occasionally feels like it or thst we might wish it to be so.
† Kirk and crew are following a Gorn (the beefy and aggressive lizard people of the Klingon Empire) vessel to apprehend it, when Captain Kirk and the Gorn captain are transported to a barren desert planet and made to fight "to determine which ship would be destroyed and which one spared". It one of the more iconic OG Star Trek episodes due to the Kirk Ripped Shirt scene.