Let me first tell you about my experience as a child:
I was raised in a polygamous family ( 1 dad 2 moms), and it was just as good for me to grow up in, if not better, than any marriage I know about in detail.
I have two moms, both prominent artists with lots of success, one from Hungary and one from Iran and they both work in the same studio. They live in Iran, where polygamy is legal ( islam allows it) but there was no religious, traditional or otherwise ‘primitive by western standards’ driver behind their marriage. (My Hungarian mom was raised in communism)-My Dad and my two moms just all liked each other.
I turned out OK, by any conventional measure you like. So did my sister. We have our own insecurities, like all other people. But by any ‘outside’ measurement we are both successful and happy. My two older sisters, who were born in a normal marriage and then my mom was added to the mix are sometimes more insecure about this.
Having two moms with different characteristics enriched my development as a person. I learned and heard more languages growing up ( German, Farsi, Hungarian, Turkish), and I learned to deal with different personalities. I believe I have a ‘lower-than-average’ sense of jealousy which has both helped and hurt me in relationships.
Would I recommend it?
No, but I don’t think it should be illegal either. It is a freedom of choice issue, just like homosexuality is. I cannot recommend homosexuality to anyone, it’s not ‘to be recommended’, people have to develop their own reasons ( or inner drive) to practice it.
Why would I not recommend it? because it is extremely unlikely that a relationship between 3 people is without trouble. My family managed to maintain this, because they thought it was worth it. There is a lot of sacrifices from each party, there is a lot of ‘frowning’ from the outside society ( especially as my family is in an intellectual class, not a working class were polygamy happens for other ‘common’ reasons), and it is very hard to keep it fair and balanced for everyone.
Knowing how difficult it is to maintain a polygamous relationship, I would think that most such relationships will fail. It requires extraordinary understanding and coordination, especially for the two parties of the same gender.
I would not practice it myself, even if it was legal. Not that I think it’s horrible to do it, because I know for a fact that one can have a civilized respectful and fair polygamous relationship. But because I think it takes so much effort to make it work, and most of the time it will not.