Portions of this post first appeared in the Comments section of Alas, A Blog. Thanks to Alas readers Zuzu and DJW for their input.
I'm not an expert on the issues but I'm becoming more and more firmly persuaded that universal civil-union recognition is the way to go. The way I see it, the case for it is threefold:
(1) Tactical: It is the one solution that offers a realistic possiblity of a consensus in the near future. Some people on both ends of the ideological do oppose civil unions, but the civil-union approach seems likely to capture the largest portion of the center. Many moderate conservatives are receptive to the argument of “getting the government out of the marriage business” and the case might best be framed in small-government language.
(2) Constitutional: It seems to me that "marriage" is an area where religious and civil institutions have become conflated. Separation of church and state demands that the law view "marriage" as nothing less nor more than a set of contractual obligations.
(3) Philosophical: The "sanctity of marriage" is, and by definition must remain, the exclusive province of the church, synagogue, or mosque. Those who fret that another couple's civil union somehow threatens the "sanctity" of their own marriage, are simply making it plain that they need to get in touch with their own religious faith.
Incidentally, I think you could make a case that any legal definition of "marriage" which privileges heterosexual unions, discriminates against those liberal religious denominations (Metropolitan Community Church and others) which do recognize gay marriage.
Extreme homophobes like the Family Research Council will oppose civil unions because, davka, they do confer marriage-like rights upon gay couples. Some gay activists will oppose civil unions because, for them, only marriage-in-name-as-well-as-fact is acceptable. The difficulty with both of these positions should be self-evident when we consider the issue in light of church/state separation: no one may deny anyone else the right to enter into a lawful contract simply because their own religion forbids it; and no one may claim that they have a right to a religious “blessing” from the State.
However, I am convinced that the religious mainstream will see this as an opportunity to reclaim the sacred rite of marriage and to place authority over sacred matters squarely where it belongs. (I have it on reliable authority that at least one Orthodox rabbi has gone on record as supporting civil unions for gay couples.) Religious leaders of all denominations have a role to play in resolving the marriage issue, and it is in their own best interests to assert their unique and exclusive role.
By presenting the issue as "saving" marriage from the godless state, civil union advocates will force anti-gay parties to either accept civil unions, or admit that they want the government involved in religious matters.
To precisely the extent that marriage is sacred, the law has no legal or moral claim to regulate it. The reconciliation of social progress with religious tradition is a process both demanding and subtle, and for good reason is just such a process off-limits to the firm hand of the law.
Marriage also implies a set of material rights and obligations, and it is with these exclusively that the law must concern itself. The "sanctity of marriage" is for the church, not the court, to defend. Universal civil unions would clarify this point beyond doubt.