It’s not the multiple partners that are a problem in my opinion (You do you. Polyamory is not for me, but no hate), it’s the many-to-one relationships. Even in cases where an immoral power dynamic doesn’t exist, you’re still setting up for societal shenanigans if it’s consistently many women to men, or vice versa, which seems prudent to avoid.

That being said, monogamy in a legal sense has probably only persisted so long because involving more than 2 people would be a massive headache for the courts lol

Probably only works in countries where one “partner” has more rights than others.