"The rights of monogamous traditional marriage are embedded all through our legal statues ..."
The only "right" of married couples I can think of that wouldn't be covered by contract law if the government got out of the business of sanctioning marriages is the right to file a joint tax return. And for that "right", married couples generally pay a penalty!
I'd rather see the government get out of the marriage business than to see its sanction extended to a relationship other than that between one man and one woman.
Well IIRC, there are numerous references to marraige, married status, husband, wife, married couples, and parents all throughout family law and divorce law, in addition to tax law and contract law. That's not a subject we all deal with very often but I think if we eliminated the legal definition of marraige or made any definition for marriage legal, then that would throw a lot of chaos and confusion into the legal system.