Maybe I have forgotten what I implied, but I don't think I was indicating that monogamy is an inborn tendency. It just makes sense, at least historically. And it has been the most popular choice.

If people choose not to be monogamous, then that is their choice. But I see all kinds of negative consequences to this behavior, compared to monagamous behavior. And why is it that it's usually the men who have all the partners, but not the women? (Just thinking about sexist behavior.)

I agree that if the couple has not agreed to an open relationship, then each partner should not step out. It does hurt.