Because homosexuality is innate and can't be changed whereas Christian doctrine is simply a view.
No-ones discriminating against a view. Just when the view impinges on people who have no choice but to be who they are.
Substitute homosexual for black and the argument is absolutely clear.
Interesting view. What evidence is there for it? Surely the fact that some people do change refutes any argument that sexual orientation is innate and can't be changed? Or else nobody would be able to change?