“Biological sex” & the gender binary are co-dependent concepts. It is logically fallacious to accept 1, whilst rejecting the other. To do so forms an impassable contradiction. It is a height of crude materialism to posit that there are 2 “sexes”, created celestially by the mechanisms of genetics.
1stly, the imposition that “biological sex” exists in 2 categories is, itself, biologically incorrect. If 1 bases the premise upon chromosomes, a multiplicity of “sexes” emerge. Not only do a variety of chromosome configurations exist outside of the XX/XY popularism, but even these 2 configurations have very little to do with the assignation of “sex”. In the United States, there are about 7,500 men without a Y chromosome – men who would in no way be regarded as transgender, intersex or genderqueer. (Intersex Society of North America, ‘Does having a Y chromosome make someone a man?‘, (2008).)
Arguments for a binary of “biological sex” also frequent an anatomical essentialism. Some1 is a man if they possess a penis. They are a woman if they possess a vagina. This is equally absurd: ‘if sex is determined by genitals, they must clearly be binary and unchangeable, right? Wrong. Genitals can be ambiguous at birth and many trans people get gender confirmation surgery to change them.’ (Mey Rude, ‘It’s Time For People to Stop Using the Social Construct of “Biological Sex” to Defend Their Transmisogyny‘, Autostraddle (5 June, 2014).) Anatomical essentialism which goes beyond this, to argue that body hair or breast size are indicators of “biological sex” is not even worthy of derision.
Biology denotes function. The only manner to reduce “biological sex” to a binary is to assign people within categories of function – ie “can give birth”, “can’t give birth”. Even this essentialism is not immune to change – menopause etc. In reality, “biological sex” is an ideological imposition, a crude materialism which serves no purpose other than to reify gender binaries.