Several years ago Dr. John Piper of Bethlehem Baptist Church in Minneapolis preached what I think is a very faithful and compassionate sermon on the subject of homosexuality and marriage.
Here's one of many helpful quotes from his message:
The other biblical reason marriage cannot be between two men or two women is that, on the one hand, the Bible defines homosexual behavior as “dishonorable” and “shameless” and “contrary to nature” (Romans 1:26-27), but on the other hand the Bible says that marriage is to be “held in honor” (Hebrews 13:4). Marriage does not produce shame. And marriage is not contrary to nature. There is therefore no such thing as homosexual marriage in the eyes of God. And there should not be in the eyes of his people—no matter what the state says.
Click here for more on the sermon, Discerning the Will of God Concerning Homosexuality and Marriage :: Desiring God Christian Resource Library