If it has a direct reference to his kink being NTR, and she is fulfilling his NTR desires she is doing it in a way he wouldn't agree with to make it NTR. So it should be tagged as NTR, even if MC doesn't see it as cheating because they like it. It's like if Anal occurs, you tag anal, I don't see why I have to cry because yall are trying to go around tags to force your fetish on others. Imagine you play a game and next thing you know Scat everywhere but no tag.
