I read Sono Mono and to this day I'm confused why people call it NTR. The childhood friend made a promise when they were children to marry him and then broke it, falling for the hero. I mean, can you blame her? She was a kid; kids make stupid promises all the time and seldom keep them. Then, she spent a couple of years next to the HERO; why in the world would she care for a stupid childhood promise made with a village boy? I'm of the opinion the problem was with the protagonist. Moreover, the ENTIRE PREMISE OF THE STORY is based on her choosing the hero over him. Was it really surprising that, at the end, she didn't change her mind and STILL chose the hero? I mean, she was in an actual relationship with the hero then; it wasn't just some dumb childhood promise. Had she dumped the hero for the MC in the end, then, and only then, we'd have NTR (netori, in this case). Her behavior was completely logical and reasonable. The protagonist is the one with serious issues.
However, considering how many people think it is NTR, I think I'm missing something, so I'm wondering whether you could clear it up for me.