In theory I agree with you. But it will make things much more complicated. In those other countries divorce is easy because the man can just tell any of his wives they are divorced, give her the mandated payment, and decide who gets the kids. He doesn’t even need a court, just say it publicly three times.
Suppose here we do it so anybody can marry and consensual adult in any arrangement. You could end up with a five person group (gender doesn’t matter) with A married to B and C, B married to A, C, D, and E, D married to B and C, and E married only to B. Now what happens to assets when D wants to divorce B and marry F? What happens if E gets sued? Can the court take assets that A brought into the marriage, even though A is not married to E? After all, if E is married to B than their assets are joint, and so are A and B’s.