Because the bible says in numerous places that it is.
And kadzbiz while Leviticus does say homosexuality is a sin, its not the best of examples to use. Some pull the Old testament law excuse, others say it was only for the Jews. The best example against homosexuality is right here in Corinthians...
So you see, the new testament(which is the covenant of christ and written for all the world) condemns homosexuality and groups it with things like prostitution and murder. Therefore there is no excuse for a christian, who should believe the bible is true, can accept homosexuality as morally correct or acceptable.
This isn't to say that we should hate gays, we should try to preach the gospel to them and help them see the truth that their sexuality is a lie from Satan that they bought into, a choice, and they can change.
And one more thing, the reason homosexuality is so disliked among christians isn't that we think its a worse sin than anything else, its that people try to make it something to be proud of. That coupled with some government support of it causes it to be a sin that we really can't stand. You don't see liars holding pride parades, hear about "out and proud" thieves, or see bills giving civil rights protecion to prostitutes, so we don't need to take as great a stance on those, because people generally agree those are bad things.