Women (a gender) is not the same as female (a sex)—a concept that dates to the early 20th century and championed by Second Wave feminist activist in the 1970s. It's the basis of transgender conceptions of the self. You can't disown it now that you don't like its logical ends.

It is ridicuous to think that no one's sexual orientation ever changes when there's ample evidence that it has. And does. Take a look at Lisa Diamond's book Sexual Fluidity for numerous accounts and some theories about why. And, no, it's not as a result of conversion "therapy" (which has never been shown to change anyone's sexual orientation). Just because you can't change your sexual orientation at will doesn't mean your sexual orientation can never change.