I know most people hate NTR.
But NTR is about losing your lover to someone else.
Actually it's even more specific. It's a narrative element for the theater, the protagonist must lose the lover to someone else and catch them. It does not even neccessarily include sex (it was created for live stage play after all), can also be a kiss or any other form of intimacy.
It's not about the other person screwing with someone else, it's about the actor being betrayed and feeling the loss.
And therefore it's also not a double standard. It's defined to happen to the main protagonist, not to other characters.
What people hate is not NTR, but other men/women who screw any girl in the game, no matter if they are LIs of the MC or not. This should get it's own label / designation, because it has little to do with NTR.
Is it unfair towards the female characters in the game? Most likely so. But I would not use that as a reason to include it. Some players don't care, others do. I doubt that any of the players here would be able to keep this double standard up in real life, in the game it does not matter. There we have no moral obligation to treat everyone the same.