As you can see, I'm on the panel for this event next Sunday afternoon. I plan to speak a bit on the issue of the entanglement of church and state, which I see as a major part of the Fog (sometimes more like a Stephen King "Mist") that seems to descend on otherwise sane and sober minds when this topic comes up.
It seems evident to me that the independence of civil and religious marriage is simply a norm -- at least if one is speaking of any particular religion or state. Marriages exist in many forms in many cultures, some with religious overtones and some not, and in some cases they are mutually exclusive. That is, marriages that might be recognized in some civil societies aren't in others, and the same goes for religious marriages and religions. There is simply no "one size fits all" including perhaps most especially the "one man / one woman" model, which is just one of many forms of marriage.
The entanglement in the US is particularly troublesome, and I imagine it to be in part a by-product of our English colonial heritage. At the time of the Revolution, English Law (Lord Hardwicke's Act) required all couples to be married in the Church of England with the sole exceptions of Jews and Quakers. This law was on the books from 1753 through the middle of the 19th century, and was a scandal for Roman Catholics and Nonconformists alike, and many quite rightly said, "I'll be damned if I'm going to get married by an English Vicar!"
It is still something of a mystery to me why the US for the most part retained this vestige of the Establishment after the American Revolution, allowing clergy of whatever sect to function as civil officials. As to the French Revolution, I presume you know that that unfortunate movement led at length to a strict separation of these powers, and in many nations influenced by the Napoleonic Code one may marry in church but such marriage isn't recognized by the state unless one is also married through the civil authorities.
I am all for freedom of religion in this regard. But at this point I am increasingly irritated with the movements by Roman Catholics and Mormons in places like California and Maine to intrude their religious beliefs beyond their own membership and meddle in the lives of citizens who want nothing to do with their belief systems. Perhaps this is payback for Lord Hardwicke's Act after all. But I've had enough of this exercise of the libido dominandi -- the root of all evil in attempting to dominate others to ones own parochial views.
Opposition to same-sex marriage on any grounds is heterosexist by definition, just as opposition to mixed-race marriage on any grounds is racist. Both are irrational, and within the next quarter century, I believe more will have come to see heterosexism's intellectual impoverishment and moral bankruptcy, as we have with racism.
Tobias Stanislas Haller