The issue in defining homosexuality as a mental illness or not is about insurance coverage. Large numbers of people go to psychiatrists asking to be "cured" of their homosexual desires. During periods when the psychiatrist's association officially defines homosexuality as an illness they can bill insurance companies for treating those people, and some of them pay. During periods when they don't, they can't. I believe it's currently a mental illness again -- was the last I heard.
The idea of defining the desire to want to be cured of your sexual desires, in whatever form they happen to take, as a mental illness doesn't seem to have occurred to anyone.