Sort of in the vein in the previous post, I'd give the fundies a choice:
1) Legal marriage is available without regard to the sex of the partners (it's available to same-sex, mixed-sex, and intersex-whateversex alike).
2) There is no legally recognized marriage, and civil unions (or some other contract) is available without regard to the sex of the partners. Such a union may also be extendable to two people who aren't in a romantic relationship but still want the legal benefits of such a contract.
3) There are no legally recognized marriages, civil unions, or any similar contract available to any coupling of people.
Of course, no particular religious denomination is required to recognize a legal marriage as a religious marriage. That tradition was around before same-sex marriage was a major topic of discussion.
While marriage is religious in some regards, it's not really exclusive to religions, so I don't particularly mind seeing a contract called "marriage" available. However, I think necessary to force society (from a legal standpoint) to not differentiate according to the genders of the partners. As to what is the maximum contract that is available to couples, I'm not really particular as long as the "covenant marriages" come to an end (since those are prone to perpetuate abusive relationships).