I've been having some rather naughty fun debating same-sex marriage. I got one Christian to admit that he doesn't follow the Bible but expects me to do so. He came up with a necessarily strained explanation as to why that's okay. Also, when I bring up Biblical teachings on remarriage after divorce and on polygamy, people keep pretending not to have heard.
In fairness, our side is not necessarily better. Some gay theists know their own holy books and are willing to discuss them. Nonetheless, most of them, at least in my experience, never cite the Bible beyond "God is love" and "whosoever believeth," and they bitterly resent having the Bible quoted to them. When someone holds their feet to the fire, they almost immediately abandon any pretense of Bible belief and start spewing enough New-Age happy talk to make fluffy-bunny Wicca look theologically rigorous.
In debating the subject, I have no illusions of changing the minds of those who most vehemently argue the other side. They are either propagandists and con artists, to whom truth is completely beside the point, or sheeple who believe in belief, to whom truth is completely beside the point. Their bigotry needs a pretext, not a basis. Instead, I hope to let those in the middle see who argues from reason and who does not. I do so because I have seen the consequences when our activists let the other side win the argument by default.