I'm an atheist so...
1.) As an atheist, I do not believe homosexuality is immoral, neither do I believe it "originated" from anywhere. It's a biological programming in organisms on the planet. Not a choice, you're born that way.
2.) This whole perception people have on atheists believing that no higher power exists is quite ludicrous. Atheism is not entirely a belief in no god. It's the merely the lack of belief of any such thing. Am I going to say a god does not exist? No. However, don't expect me to waste what I view as MY valuable time praising something that I simply have no time for.
3.) Religion did not create the idea of homosexuality. It has been around since even before many religious texts out there. It was even embraced in other religions with the Romans and the Greeks.
The thing about religions is what makes a religion tempting. That's pleasure. Pleasure can be eternal life, being reborn, physical pleasure, mental pleasure, etc. Whatever in our world is viewed to be the better pleasure over the rest is what someone will base their belief or lack of belief on.