Well for most of our evolutionary existence we were basically animals. Animals don't really care about monogamy. I mean, yes the alpha male may protect his harem and try to keep other males away but that is because nature wants the alpha to be the one to propagate as that is the best genes. Its not like if another male does her he gets jealous. He just takes her back with force.
As we have become intellectual beings, yes our desire for family and stability has begun to take over. Plus a stable home is a great thing for rearing kids. But that does not impacting nature wanting the alpha to be the one to impregnate the women. Thus why so many nice stable secure married men are raising the kids of other less stable but more desirable alpha males.
Right behind the stable doctor is this man banging his wife...
there is a powerful natural drive that tells her to seek the first guy for security but the second for her offspring.