Sex is indeed binary. Why?

The answer is because of anisogamy: reproduction through the fusion of two gametes of differing size.

Because there are two evolved reproductive anatomies to support two gametes, there are only two sexes. https://twitter.com/NatalieTheCapn/status/1282892212582285312

Sex-related traits such as hormone levels, genital morphology, height, weight, muscle mass, voice pitch and more are not binary, but exist on bimodal distributions.

But overlap in specific traits between males and females does not form new sex categories.

And the good thing is that we do not have to deny the existence of this binary system to accept trans people, as trans individuals have a diverse set of experiences, but they too exist within the binary.