The often overlooked point - no state or federal law ensuring marriage equality would apply to church doctrine. One way or the other. The state can't force any church to provide marriage ceremonies for same sex couples, nor can they stop any church from offering the ceremonies. Legislation is a civil matter. I think that's a winning tack - side step the red herring laid out by religious opponents to same sex marriages. Clearly and succinctly.
That said, it's my understanding that the state also can't block any church from advocating their doctrinal position, pro or con. I think that falls within the 1st Amendment -
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances."
SCOTUS has a long list of interpretations and re-interpretations on the application of the 1st Amendment. They've clearly taken the stand that a church can't endorse a political candidate (a standard often ignored). It seems endorsement or opposition to specific legislation is a more fluid situation, especially if it happens in a public, i.e. outside the church, setting where counter points can be publicly made.
My take - let faith based groups make their case in the public square of discourse. I don't think that's wrong.
I think conservative religious opposition to marriage equality is in itself wrong. I think the support from other religious groups is in itself right.
Kind of a convoluted reply. Your post made me take another look or two.
Agree, progress is good. And on this issue inevitable, imho. 
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