All right, I bet that everybody will tell that your girlfriend/wife ntr'd you when you find out that she slept with another dude. "Hey, did you hear? diras2010 has been ntr'd!"
That would be true if the girlfriend/wife leave permanently the previous partner, and the previous partner can still visit or be in contact with the girlfriend/wife.
In the game, I think, there is no true NTR. There is sharing, cheating, getting rid of the previous (potential or not) male partner of one of the girls, cockblocking other males...
If the MC and Alice formalized their relationship after the Merchant was cockblocked, that could have been considered NTR.
The MC have a few isolated LI and a few "open relationship" groups. But he never lose a girl. He can get on probation.
The Giant Sisters:
Hilde probably won't leave her husband.
Tinkerer will devise a way for the MC to not lose Frida.
Again, NTR means the previous male lover don't fuck the girl anymore, even if he is around and free to talk to her.