One fact supporting critical gender theory is that there is no single attribute exclusive to all members of a biological sex. Not all born-men have penises, for instance, or the same male genes, or even XY karyotypes (cf. de la Chapelle syndrome). Sex, then, is a ship of Theseus.
People either have arms or they don't. Having arms is a single attribute. Sex, on the other hand, is composed of multiple attributes, each of which is not a necessary indicator of sex.