We accept without condition the idea that gender follows from biological sex. Your body arrives in one of two options—male or female—and your gender is decided in the same moment your sex is named. But what if binary gender plays the decisive role and it is biological sex that is retrofitted to serve our need for boys and girls, men and women?
Further reading:
Sex is a Spectrum: The Biological Limits of the Binary (Agustin Fuentes): https://press.princeton.edu/books/paperback/9780691253251/sex-is-a-spectrum
Sexing the Body: Gender Politics and the Construction of Sexuality (Anne-Fausto Sterling) https://www.hachettebookgroup.com/titles/anne-fausto-sterling/sexing-the-body/9781541672895/?lens=basic-books
Sex is Not Real: The Invention of a Noncoherent Binary (Beans Velocci): https://www.dukeupress.edu/sex-isnt-real