Once again, Andrew Sullivan nails the issue with clarity and intelligence. Click the link to read the whole thing, but here is one of many money quotes:
"The reason the marriage debate is so intense is because neither side seems able to accept that the word "marriage" requires a certain looseness of meaning if it is to remain as a universal, civil institution. This is not that new. Catholics, for example, accept the word marriage to describe civil marriages that are second marriages, even though their own faith teaches them that those marriages don't actually exist as such. But most Catholics are able to set theological beliefs to one side and accept a theological untruth as a civil fact. After all, a core, undebatable Catholic doctrine is that marriage is for life. Divorce is not the end of that marriage in the eyes of God. And yet Catholics can tolerate fellow citizens who are not Catholic calling their non-marriages marriages - because Catholics have already accepted a civil-religious distinction. They can wear both hats in the public square."
I've said similar (though not with such erudition) in many blog and news comments. We can have both civil marriage equality and respect for religious views of marriage.