MG, the problem is that those that want "gay marriage" want the government and society to acknowledge it. They have the freedom right now to have some "minister" perform their ceremony and then to live any way they like. What they want is the government to enforce the rights of married couples in their cases, or in other words, they want society to be forced to recognize and accept their "marriages".
I agree with you to some degree here, but realistically, marriage is a religiously rooted concept to begin with.
The government shouldn't have a role in marriage period, which is why a lot of people support the government solely recognizing a civil partnership. You'd get married in the religious institution of your choice, and the ceremony would be religious in nature, just like a Baptism, Christening, Bar Mitzvah, etc. No legal aspect to it. If certain churches or groups don't want to perform these ceremonies for gay couples then so be it, and vice versa. As part or your 1st ammendment rights you can chose to attend an establishment that doesn't perform or recognize gay marriages if you want. That's part of the religious aspect of it and is up to individual groups.
After the marriage though, couples who wanted a legal aspect to their union could file for a civil partnership. That would allow for most of the legal aspects that are now afforded to married couples. IE, filing of joint taxes, child custody rights, hospital visitation rights, estate inheritance at death, etc. I don't see where the government should care WHO files for this type of partnership, be it a straight couple, gay couple, or just two roommates. It's a legal arrangement that can be enacted and dissolved as the parties wish.
The thing is too many or too hung up on the word "marriage" being attached to the legal (rather than the religious) side of things that they want to fight tooth and nail against any type of change there.