I guess, for me (and many other people?), ntr is just a flavor, spice for a corruption story. And as long, as there's enough of this spice - the girl experiences the feeling of betraying, the fucker(s) recall the cuck in a mocking way, etc. - that's definitely ntr.
For example,
The Student Council President Is My Sexfriend. It's written from a pov of a bull, and it's supposed to make you sympathize with him, traumatic childhood and suchlike things. But I definitely didn't associate myself with him much, and read for observing the heroine's inner turmoil.
Did I associate myself with her? Not much as well, I think.
Do you associate yourself with wood when looking at fire?
Bu there are three things you can watch forever: fire burning, water falling, and... humans getting broken.
And that's a primal gut feeling. The most important knowledge is a knowledge about humans. The true knowledge about a thing is a knowledge about the thing's destruction.