I had another one of the conversations with a conservative Christian about gay marriage. I have had this conversation a million times with fellow Christians and have no problem having it again and again. My latest iteration is probably the most succinct I have been able to be.
First, if you read the Bible from a fundamentalist, literalist perspective, its tough to have much of a conversation. But the truth is even fundies interpret the Bible, even if they don't want to admit it. Homosexuality is case in point.