Gender is a social construct established by Eurocentric standards and pushed onto non-European countries that recognized more than traditional man and woman that are associated with specific genitalia. Genitalia doesn’t necessarily cleanly between penises and vaginas, which determines sex isn't wholly binary. If it’s not hard to imagine that someone could be intersex, why is it so difficult to imagine women who have penises?

Think about it this way. Close your eyes, and imagine yourself in your bed. You wake up tomorrow morning and find out your brain has transplanted into a body of the opposite gender. Wouldn't you want to get transitioned back into the body that matches the gender you know in your heart you are? That is the crux of the trans argument, and almost everyone agrees with it when it is put this way.

There are people who are intersex, which is a biological phenomenon, and identify with either "Male" or "Female." If they can identify as a gender they aren't biologically, anybody can.