In my humble (and decidedly non-Christian) view of the matter, even if only tentatively proffered, the proper policy move should be for states to de-institutionalize marriage and leave it up to private religious affiliation.  For benefits (i.e., tax or what have you), create a personal partnership license.  A form of political disobedience to today’s tyranny of political correctness, as it were.
I would suggest this not from a Libertarian’s point of view—a political worldview I reject in favor of a constitutional republic founded on a faith and trust in Gd and limited government further cabined by the balance of powers and federalism.
Since the limiting factors preserving our founding political order have all but been removed, and possibly remain in name only or via a fleeting nod, men and women of faith ought to say to their political representatives: “Fine.  If the union of a man and woman is no longer sacrosanct in the eyes of the People’s government, the institution ought to be removed altogether from the government’s purview.  Thus, we could care less if you wish to grant a tax credit for a man and woman in a holy union or prefer to grant the same benefit to two toads croaking in unison on a lilypad.  Marriage in the eyes of the [fill in as needed: Torah, Catholic Church, Bible, what have you] is now de-linked from the Biblical abomination now called marriage by government bureaucrats in black robes.”
-David Yerushalmi, Co-Founder & Senior Counsel