How do you define "mental illness"?
Not long ago, homosexuality was deemed to be a "mental illness" as well. Try to claim that gays are mentally ill in this day and age, and everyone will call you a bigot and you will be socially ostracized.
One might say: "That was in the past! Scientists made a mistake, but today they've seen the light and no longer believe that gays are mentally ill."
But if "science" is so unreliable, what makes you think that today's theories about mental illness are "true"? Maybe in a few years they will be completely revolutionized.
In the end mental illness is defined in a way that suits the needs of the dominant classes. Gays used to be oppressed, but they've gained ground nowadays, so they changed the rules of the game and homosexuality magically stopped being a "mental illness". What a farce.