[dawnbreez]

...This still ignores examples where sexual characteristics don't actually fit a binary, though. These are edge cases, yes, but they are edge cases that occur in nature. This is why cases like the bee example are important--bees have sexual trimorphism. Drones, workers, and queens all have different sexual characteristics that affect how they reproduce. Based on the reproductive function of bee anatomy, sex is not a binary for bees.

Also, I'd like to point out--for the record--that the ability to change one's sexual characteristics, and the presence or absence of sexual characteristics, has almost nothing to do with one's gender (beyond avoiding body dysphoria). This is an important distinction to make, because while we don't have full ability to change a person's sex (i.e., we don't have artificial wombs yet), a person's gender can be independent of their sexual characteristics (as evidenced not only by trans people, but also by intersex people who identify as cisgender).