Because there is very obvious physical evidence that we ARE a BINARY sexes species. Either your a male or a female. Period there is no inbetween. Either You have a penis and testicles, or a vagina and ovaries. Just because you have a mental disorder does not mean that changes the very nature of the species just so you can feel good about yourself. What you have is a mental weakness and disorder, which will one day be treatable, it is not a gender disorder.