If someone asked you, "Why do you think same-sex marriage is sin?" you'd be correct if you replied, "Because it violates God's design for marriage." But if you take time to ask him what kinds of sexual activity he doesn't approve, you'll quickly discover that he's cherry-picked some morals over others with no basis for his choices. 

Watch how Kyle quickly realizes that he cannot endorse same-sex marriage and condemn adultery at the same time. This opens a natural door into the gospel.