As a pastor, I guess I should wade into this, even though I really don't want to. Yes, I believe that homosexuality is a sin, if you believe that the Bible is infallible and without error as I do, then an honest examination of the Bible will tell you that. If you don't hold to my view of the Bible, then that's your decision.
So yes, I believe it is a sin, but so are so many other things. That is not to say we overlook homosexuality, or any sin, but the problem is that we often single out homosexuality. I guess because people in the pew do not struggle with it, or at least say they don't. I have friends that I reach out to who are gay, and when they have asked my opinion, I have been honest with them in love and say that it is a sin.
As for gay marriage, I don't believe in it, and would not perform a same sex wedding, but sometimes I wonder if we as Christians are fighting the right war. Apostle Paul was pretty clear that he really did not care that much what the culture did outside of the church, but what he cared about was what was going on inside the church. That is why in the gay marriage debut, the one thing I am concerned about is the tendency of our culture forcing their views on those who don't accept it, so that as a pastor, I would be required to perform same sex marriages, or that by declaring homosexuality a sin would be labeled as hate speech. Just my 2 cents.