I don't get the Leviticus 20:13. It just says "If a man lies with a man as one lies with a woman, both of them have done what is detestable. They must be put to death; their blood will be on their own heads.". I can't really decipher on what it has to do with "Being gay is a sin" thing.
And seriously, I have NO CLUE why being gay is considered a "sin". I think it's fine in the first place!