Serano (to use the correct spelling - you could have checked, but you didn't) might have a vested interest in arguing that sex is not binary. However, I can't find any articles by Serano arguing this. There's a NYT article (https://www.nytimes.com/2019/06/19/us/julia-serano-gender-science-lgbtq.html) which tries to suggest that "Every person superficially appears either female or male. But once we look beneath the surface, things are far more complicated." But it does not, in fact, argue that case; Serano carefully avoids mentioning gametic production, probably due to knowing that that is the organising method of determining sex (having done a PhD in Drosophila oocytes).

As for Dawkins's expertise, your comment betrays a certain absence of understanding of what evolutionary biology involves, and the fields that it encompasses. The question of why sex differentiation exists at all is examined (and explained) by evolutionary biology. But let us know when you've written a book that provides a comprehensive understanding of inheritance and evolution. (Do check the spelling of the people you name first, though.)