I oppose ALL state-sanctioned unions because I don't believe it's the state's business to say who can and cannot be in an official relationship (hetero, homo, or all the poly/group unions). As long as the people consent, why does the state need to approve?
Considering marriage is a non-binding contract (no fault divorce), the contract isn't worth the paper it's printed on (even prenups are overruled with alarming frequency).
From the state's perspective, what does approving marriage offer? It offers monitoring and control (and a religious position), but almost nothing else. Child guardianship is already a non-marital issue. Last will and testaments along with living wills (if enforced properly) deal with most legal issues pertaining to marriage. Minor modifications to tax law deals with the rest.
If someone wants to be married, they should go to their local church, mosque, synagogue, or whatever. If they want legal protection, have a will and they should sign the proper docs (even married persons should do this). If they want a binding contract, they should contact a lawyer and get one (just as they would with a prenup if they want something semi-binding).