You are wrong. Saying something is "a sin" is simply looking at what the Bible says on the matter.

Jesus at the well with the woman caught in adultery, did not say she had not committed a sin... in fact, He told to go and sin no more. But to the people, who wanted to stone her, He said, let the person who had not committed a sin throw the first stone.

I fear, you feel empowered to judge Christians even as you criticize them judging others.