Homosexuality was considered a mental disorder until psychologists/psychiatrists couldn't come up with a "cure" so they gave up and voted it out of the DSM is 1973 but much like with transgender being replaced with "gender dysmorphia", homosexuality was replaced with "sexual orientation disturbance" until it was taken out completely in 1987.
So to recap....it's never been scientifically proven that homosexuality or transgenderism is or is not a mental issue, but at one time, they were both in the DSM as mental issues and were taken out because of political and social pressure, not because science.
from what i understand transgenderism is more about feelings rather than facts, i "feel" im not a boy, but a girl etc
but with homosexuality its not just make belief, something clearly happens physically, theyre attracted to the same sex sexually, there's a base in nature etc.
and besides, theyre not looking to cut their dick off.