I'd spend some time reading up on the history of marriage in Christian thought so the battle can be fought on enemy ground. The Catholic church sees it as a sacrament but it has a very mixed history in Christianity, beginning with Paul who does not see it as a social good, only a social necessity, and later with Jesus who has some pretty anti-family values assertions and as far as we know, wasn't married and didn't condemn homosexality. Not to mention the Old Testament which has duties to marry your sister in law under certain conditions.
The idea that 'marriage' is a religious concept needs to be directly challenged.

Anyone know the history of marriage in US law as regards it's religious or secular foundations?

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