I won't speak for Black women, since I'm white, but this is not remotely true in general. In lots of poly and otherwise open relationships, women are involved with other men. There's an entire subcategory of consensual non-monogamy know as the "hotwife" where the entire point is for the wife to fuck different guys. Most swinger women are having sex with men.

But that being said, I agree that we don't really know the inner workings of anyone else's relationship and assuming that we do and that it conforms to traditional parameters of marriage is absurd. I think people need to stop judging things they actually know very little about and go mind their own business. Let them figure out their lives and live them as they see fit. It doesn't affect anyone else.