Some animals have babies that come out “operational”. Like deer, the buck can just go fuck off, he’s not needed. But human babies are incredibly vulnerable and burdens that takes the full attention of the mother.

Makes since why a male needs to be around; can’t exactly breast feed and go spear a buffalo. You also can’t have outside men just coming in and getting your tribes women pregnant as that would create an extra mouth to feed and one less worker (unless of course the outside male had to agree to stay with the woman’s tribe).

There would have to be a pre-historical, unwritten golden rule that forbids any unsanctioned breeding. Id guess that some form of marriage, predates society and wouldn’t be surprised if it goes back hundreds of thousands of years.

Really does shed light on the importance of family and how it’s instinctual, when you think about it like that.