but as far as natural occurrences goes homosexuality must be considered a mental abnormality, no?

Depends on if you consider homosexual behaviour as something unnatural.

If the answer to the question “is homosexuality a mental abnormality” depends on if you consider homosexuality natural, that would mean that being unnatural is a condition of a mental abnormality, which, since people are born with mental illnesses and not resulted from human activity, would also exclude mental illnesses

Am i misunderstanding something?