Being gay isn't a sin. However, it may be possible that judging people for being gay is in fact a sin. I think we've been taught a lot of things about the bible that are misleading. What changed my mind was scholars like Pastor Jennifer Wright Knust.
Basically, what the bible says about sex in general is not understood and outside of that larger context it is easy to point fingers at people who are different and much harder to look at oneself and understand right action in this important area of life.