Binary sex refers to the societal construct that says there are two sexes (male and female) as determined by chromosomal makeup with females having XX sex chromosomes and males having XY sex chromosomes. The flaw in this logic lies in the existence of different chromosomal configurations such as XO, XXX, XXY, and XYY in addition to XY females and XX males.
More About Binary Sex
A common argument is that while binary gender is a social construct, binary sex is absolute. However, this overlooks the various chromosomal possibilities that actually exist beyond XX and XY. Looking beyond the idea of "sex" as two mutually exclusive groups with everyone fitting into the categories of "male" or "female," allows room for people who may not fall into either category, such as intersex people.