Sex, on the other hand, is quite real. And sex is absolutely binary. There are many secondary sexual characteristics which are not binary (though often bimodal), but none of those other characteristics are themselves sex. The binary nature of sex is about a billion years old. It is a DEEP evolutionary adaptation. There are organisms that reproduce non-sexually, but for sexually reproducing species, it is always, always binary. There are no exceptions on earth. Even in cases where a species is a true hermaphrodite, they can be both sexes, but they cannot be a third sex. There is no third sex, because there is no third type of zygote. And there are no mammal species which exhibit actual hermaphrodism, so none of those complications (which still don't change the binary nature of sex) even apply to humans.