I think, OP, you are asking about the natural family state in terms of an anthropological sense so I’ll answer (since I majored in anthropology in college but didn’t graduate
The natural sense, in terms of what life was like 25k or so years ago, is that people lived in large family groups. Not all were related, but most were to some extent (distant cousins, etc). While it’s impossible to say with any confidence that men and women were monogamous to each other, anthropologists can use current tribes of people that are relatively untouched by modern civilization as examples. People are, in general, monogamous. The real main differences between ancient peoples and us today is the extended family groups, and for that, I think they were better off than us.