I have to agree, I do not get why some people get so hanged up on the whole NTR thing... that scene wasn't even NTR, it was cheating, yes, but not NTR. As far as I understand it, for NTR to exist, there has to be a component of humiliation that the character actively seeks, like he hates it but can't do anything about it and can't stop seeing it.
Here, the MC saw what his GF and father were doing, didn't stop to revel on it, didn't behave like a cuck, he immediately cut both of them out of his life and moved on.
I also prefer to be shown than told, after all, this is a VISUAL Novel. And this is a very important background point of why the MC moves, why he cuts his father off his life... etc.
Now, if the GF was to ever return and we were not even given a choice to take her back or not... then yeah, I would understand being upset. But as far as I have understood, the GF will never come back (and, if she ever does, I hope we have the option to tell her to GTFO).