I haven't seen any convincing arguments that polyamory does not reduce to a harem system[1]. 8000 years ago, 17 women successfully reproduced for every man who did[2]. In cultures with polygamy, this can be an advantage in warfare as there is a large body of excess young men to use as cannon fodder. But you could hardly argue that this compatible with the stable sorts of cultures most of the users of this site are more comfortable in. Giving men from the bottom 95% band of attractiveness access to reproduction via monogamy seems necessary to unlock the amount of productive labor needed for a technological society.
> I haven't seen any convincing arguments that polyamory does not reduce to a harem system.
I haven't seen any convincing arguments that it does in humans. If women and men are equally polyamorous, that actually means more sex available broadly speaking, and when you aren't dating to reproduce or settle down forever, your standards can be looser as well.
Now, I don't believe polyamory will ever be dominant, or even mainstream, I just despise the "loser need sex for cohesion" argument.
The harems are already here. The most attractive individuals among both men and women get a lot of attention from people attracted to them, and thus have their harems and can sleep around a lot (if they want to).
Say the "successful on Tinder" group of 5%-er men who go to the gym nonstop, and have won the genetic lottery. There is also the "influencer" group of women with countless thirsty men in their DMs. Not every woman manages to become an influencer, but many are extremely beautiful.
Are people in these harems happier being delulu than in some monogamic relationship? I'm not really sure.
Industrialized society has managed to deliver lots of high quality products by replacing the human component with machines... relationships are something entirely between humans however, so they are hard to improve with industrialized processes. Until robotic girlfriends/boyfriends arrive of course :).
Women have strong biological incentives to have fewer partners, even with birth control - they are far more susceptible to sexually transmitted diseases, for one, as well as violence, and more partners means more risk of any one partner being a problem on that front. And all it takes is one.
If resources are constrained, it’s a toss up if it’s better to have one dedicated partner with a strong bond, or multiple partners with weaker bonds. Depends I imagine on a lot of other dynamics, and if there are kids involved.
There certainly are dynamics where many weaker bonded partners is worth the risk, or prostitution wouldn’t be the world’s oldest profession.
For an example of what a society like this looks like now, there are a number of Muslim societies (and a few hidden corners in the US) that legalize polyamory.
Notably, they follow pre-birth control setups and are male centered (one man, many wives), and do indeed exclude large swathes of men from the economy and relationships.
One woman, many husband type setups are quite rare outside of nomadic/pastoral communities.
As to with birth control? Harder to say.
Many<->Many setups seem to have a lot of historical precedent, but mostly in societies without strong property rights (nomadic tribal) as they tend to be unstable over time and unpredictable. No large scale societies exist like that I am aware of.
[1] https://en.wikipedia.org/wiki/Harem_(zoology)
[2] https://psmag.com/environment/17-to-1-reproductive-success