People, please, no, Japanese (NTR) with the father fucking the mother, no, if they were separated and the MC was there having a relationship with her and she had that happen, then yes. As far as I know, father fucking mother is normal, otherwise where would we have come from?
If she doesn't stay permanently with the MC and stays married, that's not NTR and if she has sex with her husband, that's not cheating, because she hasn't ended her marriage yet.
Another person within the MC's Harem or outside of it may have a lesbian relationship, and that is not NTR, because even if they are among them it is just to have fun while the MC has fun with others