The real question is, "Why is the Government involved in marriage at all?"
Really, why? What reason does the government have in saying who's married or who's not?
I'm sure people can figure out how to make babies without the government. People can figure out how to live together without the government. What reason do they have?
With a few minor changes in existing programs and laws,(for example everyone would have to file their own taxes and if they didn't work at all they could assign their deduction to someone else) the government would be out of the marriage business, then it would become a church thing and you'd be free to "marry" anyone or any number of people you want.