For argument's sake, let's say Transsexualism was a mental disorder.
Explain why in over 50 years of trying, no form of psychotherapy, drug therapy, electro-convulsive therapy, or brain surgery has EVER alleviated the condition?
Even the most pervasive, insidious mental disorders show some response to one of these forms of treatment.
Yet amazingly, when you transform the BODY of a Transsexual person, all their gender problems go away.
MAYBE... just maybe... Transsexualism is actually a PHYSICAL CONDITION. MAYBE it is a A DEVELOPMENTAL MISTAKE, and that's why a PHYSICAL CORRECTION works as the solution.
Even someone as simple-minded as you appear to be, should be able to grasp this.