I believe people should have the right to do whatever they want as long as it is not harming anyone else. Although I think it is harmful to society as a whole, as a Christian, I am separate from society and its popular views. Why should I care so much?
Also, my faith is a choice. Christianity is a choice. If I force God's law onto other people who haven't even accepted Jesus, how is it a choice for them? All they're going to see in God is an angry, controlling ruler with a lightning bolt in one hand waiting to smite anyone who refuses to follow Him. Instead they need to see a gentle, loving God regardless of if people believe He exists. The idea itself is a good enough start, but others like me can't promote that while this country is bound by laws that only create further rebellion and hatred for my faith.
Gay marriage should be legal. Judgement isn't mine to bestow.