Segnosaur
Penultimate Amazing
It seems like the problem is (at least to me) is that you seem to be minimizing the issues that the government would need to deal with.X and Y want to form a relationship. This includes things like co-habitation, sharing of assets, giving a person proxy rights to make legal, financial and medical decision for another person, inheretence of assets upon death, things like that. And honestly I think things like that, the purely legal stuff, is all the the government interest in "marriage" should be limited to.
Yes, issues like proxy rights need to be dealt with, but I think that by itself can be a major problem (especially if there are cases where people don't define a proper 'marriage contract'.)
Then there are a few other issues:
- immigration... if someone moves to another country, most destination countries will allow them to bring in family. What if the person is part of a polygamous relationship... can they bring in ALL members (spouses of spouses)?
- Taxes and benefits... these can be impacted by marital status. This will get a lot more complex if you have more than 2 people in the relationship.