Maybe because if I get schizophrenia or diabete or Cancer I need treatment to stay healthy or indeed alive.
This does not apply to homosexuality.
That is why it is no longer classified as a disease. Duh.
Technically wasn't it classed as a mental disorder of some type.
I am not sure I have ever heard of a definition of mental disorder which is divorced from someone's idea of what normal behaviour and mental states is. Some of the symptoms I have seen listed for some personality disorders for instance, seem to have this about them.
I suspect that homosexuality was declassified as a disorder mainly because the Zeitgeist changed and it was no longer seen as "abnormal" behaviour. Nothing to do with new biological or medical knowledge coming to light, as such.
Necrophilia is still a mental disorder I believe, but then that is just weird and disgusting isn't it.
As for whether adults should be allowed to change their sexual orientation. In theory if there was a safe way to do this I would probably say why not.