The Right loves to invoke religion, and unfortunately we on the left have let them get away with it for too long. It's been a skillful move on their part, because it has led to a lot of general anti-religion bashing from our side, which makes THEM look like the victims. And their current favorite meme is "religious freedom".
The first point to make is that Religion is not of one mind. And the Roman Catholic and Mormon hierarchies do not speak for all religions. Indeed, they do not even speak for their entire communities; Roman Catholics in the pew support gay marriage at about the same rate as other Americans, despite their bishops. Moreover, there are lots of faith communities that are welcoming to various degrees to gay families and gay marriages, and either fully inclusive or getting close to it. These include UCC and the Episcopalians and the Reform Jews and many others. The Episcopalians, the tradition next to the Catholics with which I am most familiar, have a gay, partnered bishop, and many faithfully partnered gay clergy. So, the right wing fundamentalists do not own religion. (See This post for more on the clergy.)
A corollary to this is that there are substantial numbers of GLBT people of faith; many of the religious denominations have active GLBT groups working within the religious structure to change things. I mean, do they really think that there aren't gay people in the pews?
Thus, the opposition cannot speak for all religions, and must share the public sphere with other faiths, as well as with those of no faith. This is really fundamental. Prop8 allowed a single view of religion from an unholy alliance between the Mormon and Roman Catholic churches to dominate the discourse. More reasoned clergy views were ignored. And there are a lot of clergy who are pro gay rights. Some are part of the HRC Religion Council or the California Council of Churches or the coalition group California Faith for Equality.
So the first conclusion is that some religions actually SUPPORT gay marriage. Therefore there is no single "religious viewpoint" on this issue, and we must strongly, but respectfully, point out that the vocal opposition does not speak for all religions.
Myth one busted.
Now, an important part to this is that it does not serve our side to attack those of faith generally. There are lots of progressivee faithful folks who are on our side (some of them hang out at Street Prophets, which is part of the Daily Kos family). Beware the over broad brush. By letting the other side paint us as "anti-religious", we lose. Far more effective to recognize religion is really on our side. After all, Jesus didn't hang out with the Pharisees and the priests. He was with the ones they reviled.