Biology isn't the only field of science? Social sciences also studies transexuality.
Second, biologically, everyone is born with a "instruction manual" on how to make a male AND a female body, but our combination of chromosomes ends up deciding which way to branch of to (not to mention other factors in the case of intersex people), and while it's still early, biologists have found evidence of the brain and the body of individuals having developed in different directions.
Indeed, science doesn't care how you feel.