Only for the last few decades, this has almost always been a taboo across all of humanity, this high level promiscuity you speak of. Hardly a normative experience across space and time.

It only really became taboo due to religion and the associated oppression of women.

Humans, for the vast majority of the time they existed, were largely free range. Lots and lots of sex. I assure you, hunter gatherers were not monogamous suburbanites who attended their white Christian church.

Well, OK, that's true. I was wrong with "always".

It's only been the norm since we've had effective birth control, decent pregnancy and early-life health care, cures for most serious STDs, the notion that neither women nor children are property, what would nowadays seem like a reasonable amount of individual physical and social mobility and independence, certain knowledge of paternity, a less inheritance-based economic system where certainty of paternity isn't as overwhelmingly important anyway, and whatever else I'm forgetting.

But those are the new normal. Or at least one may hope they'll stay normal. And they've definitely been more or less normal throughout the lifetimes of anybody who's in this forum.