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.
Yes, there's actually a name for this kind of total skepticism: pyrrhonism. It dates back to the Ancient Greeks, predating Godel, Heisenberg and Foucault by over 2000 years.