That analogy would work only if we assumed that the gay and het unions were of different kinds. But I explicitly argued for a two-stage approach to ANY union, both gay and het: the civil union stage sanctioned by courts, followed by the optional moral union sanctioned by whatever moral authority one chooses. This way, gay and het civil unions would not be separate, but in fact, of the same kind.
Perhaps I am misinterpreting the US legal system, but such two-tier system is already in place. When I got married in PA I first had to go court to bring all necessary documentation and having a license issued, which then I took to an acceptable to the Commonwealth "master of ceremony" who administered the vows (which in our case was a district justice, but it could have been a religious figure, had we so chosen).
What is more, Poland had a similar system - the only legally recognized "marriage" was a civil union sanctioned by the local Office of Civil Affairs, and that union could be optionally followed by a religious ceremony. The reason of establishing such union, and the Office of Civil Affairs in general, was to do away with separate (and not so equal) marriages administered by religious authorities. In fact, in the pre-socialist Poland, all civil functions (marriages, births, deaths, etc) were administered by appropriate religious officials (Catholic, Jewish, Protestant, etc.) and one could not legally marry outside organized religion. So for example, if a Jew wanted to marry a Catholic, one had to convert (you guess which). In that context, establishing a civil union administered by the state rather than a religious authority was in fact abolition of religious apartheid. Needless to say that many people did not recognize such "civil unions" as "marriage" unless they were followed by a church wedding - but such wedding was a pure formality that did not carry any weight (i.e. in order to re-marry all you had to do was obtaining a civil divorce from a court; of course, the Catholic Church would not give you one, but that did not prevent you from entering another civil union, you could even get blessings from another church if you lied to the priest, there was no legal penalties for that).
I also think that creating a two-stage system - civil sanctioned by state, and optional moral - for all couples (gay and het) is politically feasible, because it would gibe the gay couples legal rights which they do not currently have, and at he same time was palatable to the right wing fatheads on the grounds that it would "remove the state" from the "sacred" institutions of marriage. In other words, whether you are a same sex or hetero couple, the state would only give you a civil union license, accompanied by certain legal rights. But if you wanted to enter a 'holy matrimony' , you would have to go to a sky pilot of one sort or another.
Wojtek