I think gay marriage is a bit of an anachronism for the following reason:
There is a difference between believing in "God" and believing in religion. Both are man-made constructs, but whilst there is mileage in believing in "God" - at least in terms of believing in a creative force that created something from nothing - even if it is in questioning what created the quarks and subatomic particles that existed prior to the Big Bang as a foundation to some form of spirituality.
Religion on the other hand is a man-made construct that was designed by people to meet social, economic, political and cultural needs and is very much open to debate. The "teachings" themselves are dubious as they are open to interpretation - witness the ramblings of Catholic priests before the Bible was translated into English so that the people could make their own minds up about what it means - or present day interpretations of the Koran particularly for the benefit of Urdu or Indonesian speaking punters who can recite parrot-fashion but don't speak Arabic. Such Priests and Imams could tell people almost anything and there are numerous examples of the same piece of literature being interpreted in multiple ways.
So the issue for gay people is that most interpretations of teachings are homophobic. Presumably the gay people that want to get married 1) believe in "God" in one form or another - no problem there and b) are part of a religion - after all marriage is a religious ceremony created by humans to fulfill a socio-cultural need (perhaps to support the concept of family stability?) and that's where the problem lies.
I don't see why a person would seek a stamp of approval from an organisation that traditionally disapproves of the very act being approved. Where's the value? I feel sorry for gays who are conflicted by their own belief structure - must be helluva confusing for them.
It's a bit like British Asians wanting to join the BNP.