If the Male MC or the FMC's boyfriend/husband isn't made aware or ever finds out, it's just cheating, not NTR.
IF you classify that as NTR every game with a Male MC cheating on his wife/gf would be NTR too, but no, they never get marked NTR, it's just because silly people have to think of themselves as the husband/boyfriend in every game and the one that is getting cucked/NTr'd is them not the NPC in the game. That is very unhealthy behavior.
It's possible for NTR to exist with a completely oblivious MC, what hallmarks it being NTR is the POV involving the male lead as a humiliated loser whether knowingly or not. I agree not all cheating is NTR, but it's pretty easy to tell when the intent is NTR, because they all have the same gross hallmarks of focusing on the denigration of the male lead as much as possible, and emphasizing that they will always be betrayed by pretty much anyone they're close to.
But NTR fans gatekeep the tag harder than professional goalies blocking balls in sports, desperate to shovel off any NTR that doesn't meet their purity test onto the unwitting users who might filter out the dreck. I remember psychos arguing that texts from a girlfriend very openly focusing on cuckold-play because they'd broken up or because there wasn't explicit scenes implemented yet in the dialogue that it didn't count.
The problem being, anybody who isn't hyper dialed into playing every piece of cuckold media and wanting only the purest of cuck-shit to get off to have a completely different line drawn in the sand from somebody who hates sado-masochistic infidelity of any kind, but don't mind sane down to earth cheating plot lines that aren't hyper exaggerated SM humiliation fever-dreams characteristic of NTR media.
