If you girlfriend, wife or possible person with whom you are establishing a relationship has sex with another person, I think it does not matter if it is a woman or a man it does not change the fact that she / he is cheating on you, not because she has sex with another woman changes the fact that it is Ntr. (I do not speak of that scene in general since I did not see it, but certainly imo would be ntr, although I do not know how much progress has been made in their relationship) It is not the first time in a VN that the Ntr is between 2 women since in the end it is the same (Hell I remember a VN that started with Mc arriving at his house and finding his wife being fucked by another woman with a strapon
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