Ethan keeps saying he needs to not judge things by human standards and morals, but even from a demons perspective, it seems strange to me that they would culturally accept sharing their wives, even if that wife is a succubus, and the one they are sharing it with is their son. Why? Because three of the Cardinal Sins with which demons are meant to embody are Pride, Greed, and Avarice, and neither of those three sins are particularly condosive to sharing.
So it's extremely weird to me that they would just shrug it off, like, "oh, she's just feeding our children, it can't be helped," especially when the level of intimacy and passion clearly goes far beyond the level of "just feeding." I mean it would be one thing if they just begrudgingly/reluctantly accepted it, but they seem to not care at all.