SCsoccermom makes a relevant and interesting point about civil unions. i'm starting to think someone important should undertake to explore or at least explain what 'marriage' is: a religious union foremost or just a legal one?
if it's simply a court-sanctioned statement of two people's intent to live together - and so enjoy the tax breaks and various other perks - then what opposition can the Church maintain? does God really give two shits about the size of your personal exemption? of course i'm sure they would still denounce same-sex relationships but that's a whole other story.
if, further, marriage is to confer the acceptance and blessing of Him upstairs, then it's quite reasonable for the religious lobby to say 'No, He's not cool with this.....look, it says so here in the book'. While I'm sure there are plenty of gay people who follow the teachings of Christ (that is to say, the peace-and-love New-Testament stuff), I can't see how you can be both gay and a member of an established church which condemns you as a sinner.
there needs to be a very clear delineation of what it means to be married. it should be the case that a heterosexual, atheist couple are treated precisely equally to a same-sex couple in the eyes of the law. call that civil union if you want, it sounds grander than 'marriage' anyway. but that has to be separated as firmly as possible from the whole church service and so on.
ps sorry if i've completely failed to grasp the issues, i'm not in the US. and i'm intellectually lazy.