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.
Actually, thats debateable ..the words "malakoi and arsenokoitai" are the words that are accasinally translated to mean "homosexual", now in days. But you also have to relize that, that translation is missleading, "Makakoi" has been interpreted many ways, In Mathew 11:8 it was translated to mean "Soft" when refering to clothing, ....
In other words, the translation is ...if-y at best
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.
Being Homosexual or Bisexual is not a choice,
People would rairely ever choose to be in a situation where they are constantly rituculed, and shunned against...you cant help who you love, or who your attracted too,
What your basicly telling us to do, is MAKE homosexuals ignor there feelings of love and affection, and be with someone they DONT love ..or even lust after, because it makes you feel better.
I would think God would much prefer people to be with the person they love, after all ... god IS love.
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.
Prostitution nor thieving is somthing your born with, thouse are for the most part choices you can make to become, your comparison is rather ridiculouse in that respect.
And not ALL homosexuals are proud of who they are, they except it, but thet dont go around telling everyone there homosexual, mostly out of fear.
The people that DO go to the parades are there more to support other homosexulas and bisexuals, telling them its ok to be who you are ..so in that respect its more like a black pride parade/event.