You might just recall that someone once suggested, "Render unto Caesar that which is Caesar's. Render unto God that which is God's." Following that advice we Americans might really separate Church from State and stop confusing two different situations: civil union and Holy Matrimony. Following the example of other countries we might leave Holy Matrimony up to the individuals concerned and to the (religious) community willing to recognize their bond as such.
Civil union should be solely a secular affair: let it deal with legal responsibilities, property, and privileges of worldly association. Whatever responsibilities and obligations Holy Matrimony entails should remain solely within the community that recognizes it and not involve the State in their enforcement. What the State should enforce is whatever the laws with respect to civil union entail.
Americans, like citizens of many other countries, should be required to go through an independent process of civil union, if they want the legal benefits of civil union for the matrimony their religious (or other) community recognizes.
So what's the argument about? It is little more than a distraction that diverts our attention from the fact that many Churches in the United States have traditionally used their secular influence to dip into the public treasury -- and into the pockets of others outside their congregations -- particularly by avoiding paying taxes, even when their "religious" activities compete with secular businesses. The way things work today, the State grants civil union privileges to the participants in matrimonial ceremonies of some -- but, prejudicially, not all -- religious communities. In the name of justice, that should be stopped. And those privileged denominations should be made to pay taxes besides.
If the Power Player churches who have traditionally exercised secular privileges would be forthright enough to say, "It is a truth of our Faith that those outside our congregation are rightly compelled to render their worldly possessions for our support," we might look less askance at their hypocrisy, even though such has been the Faith of every oppressor humankind has had the misfortune to experience.
For more on State-Church issues see, Religion, Intelligent Design and the Public Schools: serving God to Mammon?