Marriage is traditionally a method of transferring property and managing inheritance, of ensuring that the right person gets your shit when you die.

Adding a third person breaks a TON of assumptions built in to marriage laws. I've got nothing against it - in fact, I think multiple marriage seems a perfectly reasonable thing to let people who want to do it do. The catch is, it *will* require a complete rewrite from the ground up of a metric ton of law, to eliminate all of the situations where the inherent assumption of two-and-only-two breaks something.

As such, polygamy and polyandry are MUCH harder to work than jsut letting any two adults be the two adults in question.