And in a world where the relationship is procreative, the multi-decade long shared task demands stability.
I've seen women and men with a 'hierarchy' of partners, and they all know it (these were typically D/s relationships as well, so it was at least consentual [0])
I've also seen cases where people just like to have a routine to go through. My partner and I are no longer practicing poly, but even when it started they were, aside from special occasions, just a one night a week thing. It worked out well because it was a night I worked late, so we couldn't do anything anyway.
But there were times it was a little unbalanced one way or another, really though as long as you can communicate, it can get worked out.
As far as procreation/etc, Aside from Mormon and other polygamous cultures/religions, it doesn't NOT work out.
Or, I've seen scenarios where a polyamorous relationship allowed couples to have children where they could not otherwise.
I think harem builders create lots of animosity tho.
[0] well, parts were. Others were abusive.
I used to think this and remember my friends and I starting to have this idea that marriage is a social construct in high school and college as we tried on our philosopher hats. A little later I read an interesting book called Sperm Wars [1] that largely tried to line up the results of scientific papers and behavioral surveys with attitudes and narratives about sex. They had what was a compelling argument for me that the cultural ideas behind monogamy are a social rationalization for what we’d be largely doing anyway. In other words, they argued that from as far as they could tell, we tend to have serially monogamous pairings, and the culture and religion views on marriage are just after-the-fact explanations for it, not the primary cause of it. There are, of course, lots of other species on our planet that also tend toward serial monogamy, and don’t have the same social constructs that humans do.
[1] https://en.wikipedia.org/wiki/Sperm_Wars