I guess NTR works similarly how people use the word "cucked" sometimes despite the fact that, to my knowledge, cucking is a sex specific thing whereas NTR would apply in the situations people use it like someone cucking you out of your sandwich lmao.
NTR is essentially 3 genres, shoe horned into one(cuckholding). Netorare(contra netorase\netori) has more to do with emotion\feeling, as you're meant to identify with the victim(the one being NTR'd). It's not supposed to be a fun "swapping" experience, so to speak.
This is what bobdillan was refering to. There doesn't need to be any actual connection or relationship between the protagonist and his love interest, for it to be Netorare(though it often is), only the concept of the protagonist having feelings toward the woman being swept away. Hell, NTR doesn't have to be non-consensual at all, and the female love interest can simply just have gotten a boyfriend.The important thing is, again, the protagonists viewpoint and position.
Example: Say protagonist is a shy introvert hikimori. He doesnt' go out, doesn't interact with the cure girl next door, and all he does is watch her from afar. He's madly in love with her, but unable to actually do anything about it, or interact with her. But, through his immense stalking, he knows that she is single! She's available for his affection(irregardless of how realistic or unrealistic this is). We get taken into the mind of this protagonist, made to identify with this poor sack, then, the twist. Suddenly he sees a strange man entering the house! A little timelater, he hear sounds, and then, he can spot through a little gap in the curtains, the unmistakenly sight of lovemaking! His world is crushed. All the fantasies he had of asking her out, her saving him, them dating, then getting married, all of this stolen by this mystery man! This evil, horrible man(that's simply her new boyfriend, that's probably a nice dude, but we don't see him from 'their' perspective, we see it from mr hikkimori's perspective), corrupting the love of his life infront of him!
That is NTR.
So in spirit of this, sandwich NTR is "possible", lol.