Based on my experience, humans are mostly non monogamous but marriage like religion serves to preserve social order. Obviously many people prefer monogamy. What bothers me is people who adhere to group norms with a sense of superiority and self righteousness as if each person doesn't have the right to choose alternative lifestyles.

I completely do not get polyamory, but if it works for others, I say go for it. Just not for me. One person is enough work, ha ha!

It is illegal to marry more than one woman or man, but nothing in the law prevents you from living with several or having multiple partners, married or not.

While more common in men than women, I have known several married women who had multiple partners, one was married 3 times and "cheated" on all three husbands, although she and her present husband of about 25 years went into the relationship as an "open" relationship.

Some creatures are more monogamous or poly than others. I don't see why this can't apply to people as well.

our biology shows us to be mostly monogamists.

No, monagomy is unnatural but perhaps culturally necessary. Otherwise those commune of the past wold have continued past 20-30 years

I like poly because it gets rid of unnecessary conflicts. Like if you're in a monogamous relationship and you cheat, it's a betrayal of trust. Whereas if, from the start, everybody consents then no trust is betrayed because everyone knows what they're getting into.