The best argument I've ever heard is that marriage should be abolished as a civil institution and all tax and citizenship benefits should be transferred to civil unions, which are wholly separate from marriages. "Marriage" should be whatever you and/or your church want it to be; "civil union" should be between two consenting adults.
It's a wholly semantic issue. Marriage is part of law, because marriage is a cultural thing, and a human thing; religion simply codified it. Pair-bonding is older than humanity. The problem being, of course, many people believe not only that religion invented 'marriage,' but their religion in particular. I agree with you. They're wrong, as a matter of fact. But if it makes them less antagonistic, let them have it. I value people more than words.