Gender and sexual orientation are completely unrelated. My sister was once my brother, who was straight, married, and a father. She's now a lesbian transgendered woman. In these cases physical gender does not match "mental" (for lack of a better term). These 2 things are determined at different points during gestation - almost always they match but sometimes not. So is it a mental issue or a physical one? They can change their physical one and this is the way many decide to cope with the issue.