Absolutely nothing is actually ever proved in science.
Everything in science is just theory—not just almost everything, but absolutely everything. Scientists observe the world, make up a theory about something, and then test it against their observations... if they match up at least 99.9999% of the time or so it becomes an accepted scientific theory. But nothing in science is ever actually proved. Not Newton's laws nor E=mc². If a theory matches real world observations for 500 years without a single exception, that still does NOT prove it. Why? Because if that did prove it, then you could say for example one can prove that no whole number is equal to zero, because you could pick an infinite number of whole numbers so that none of them would be equal to zero...
So yes indeed, everything about transsexuals is "just" theory too. But if they would only believe something is true if it's proved, then the only things they should be believing would be conditional statements such as "if A is a whole number greater than zero, then it is not equal to zero"...