Science

Polyamory has benefits, but it also has big problems.

One is a scaling problem. Let's say you have a couple. Then add one person. Now instead of one relationship, you have three to worry about. Add yet another person, now you have 4 relationships. Add another person, now you have 9. Have each new so get a so, now you have 30.

Relationships are hard. Most people can't manage 1, so the idea of managing 3, or 9, or 30 is starting to get really difficult. You can split people off and say "hey, I'm going to just have time with girl 1 today and girl 2 tomorrow", and that can set up a sort of firewall, but there's a bit of a resource problem there where there's only 24 hours in a day and someone's going to feel left out or someone who needs more support won't get it.

The resource thing also hits in other ways. A lot of women want children eventually, and ideally children require resources -- space, time, money. In a monogamous relationship, a woman can monopolize a man's resources, whereas under polyamory she needs to share. "Sharing is caring" rhymes so it must be true, but the data shows that a child's quality time with their father is directly responsible for positive outcomes, so in that case maybe sharing isn't caring after all.

On the topic of kids, there can be a real problem if a woman gets pregnant. Whose is it? Now maybe one of the men steps up and says "it's mine" even though it isn't clear that's the case. What if he doesn't though? A shared responsibility often becomes nobody's responsibility.

Honestly, you can make it work, but it's hard mode. It's much more difficult to make polyamory work than monogamy, and many relationships that should die get drawn out by people who think just adding more people will fix things. In reality, a strong polyamorous relationship is based on relationships that would be strong monogamous relationships.