The further we get from religious orthodoxy, the more diversity in human behavior we see arise naturally. Just like many people are wired for monogamy, at least some of us are wired for polyamory.

I can absolutely see how this behavior could be a more reliable way of passing along diverse genetic material in hunter gatherer societies. Raising children was more or less communal anyway, so the identity of the father mattered way less (plus I assume many of the fathers died young).

It actually carries a lot of benefits over pair bonding in these scenarios: less conflict for romantic rivals given the expectations, addresses the fact that men were far more likely to die young, and children without fathers were simply raised by the women of the village by default until the boys were old enough to hunt. Some level of genetic diversity would be provided by mixing pairs, which was likely augmented by exchanging or abducting women with other tribes.

Again, monogamy is also evolutionary useful once we organize into bigger societies — if land / property ownership becomes a thing, knowing the identity of the father presents a way to ensure resources are consolidated in eldest sons to ensure there is a line of descendants with a much better chance of passing on their genes — with less conflict due to the social norms. The levers that ensure genetic material is passed on changed, and I also believe monogamy is innate for a lot of humans. There is probably also a middle group who, in the absence of social norms either way, would be okay with a polyamorous situation to various degrees.

In modern society, patrilineal inheritance is no longer the norm, so the advantages of monogamy are reduced. In fact, monogamy carries some disadvantages relative to polyamory that I think aren’t obvious at first.

My family are TINKs (triple income, no kids). This enables a massive jump in standard of living without having to sacrifice our passions — my husband and I have high-paying tech jobs while my wife is a social worker. Once we’re ready for kids, we have an extra parent to pitch in on household chores and child rearing. I’m genetically infertile anyway, so if my husband were monogamous with me he wouldn’t be passing along his genetic material. If he were monogamous with my wife, their earning power would be severely reduced. Bonus if you’re “50/50” bisexual — there’s no need to have to choose which half of your identity gets erased to avoid “cheating” on your partner. When one of us just isn’t in the mood for sex or is traveling for work, it’s way less of a problem. I’ve noticed a huge influx of well-off, “socially mainstream” people into the poly community in recent years, which tells me this idea is getting more popular as taboo around sex start to subside (tho most of us are closeted at work to avoid judgment).

That said, it takes some pretty evolved emotional intelligence and communication skills to balance 3 sets of interpersonal relationships. It’s taught me to be able to be radically open about very personal things that initially seem difficult, but I also have to compartmentalize my life at work — we agreed my husband would be my “official” spouse at work as my wife has zero interest in networking with a bunch of techbros anyway.

TLDR: evolutionary diversity is amazing in how it has provided the basis for the survival of the species across many varied types of society over a long period of human development. Polyamory is one of those where the taboo is starting to disappear.