Sex is role-dependent throughout nature, and describes a series of traits that are not linked. Nor can any single trait determine whether a person is male or female — not genitals, chromosomes, or hormone levels. As many as 1-in-500 males have two x chromosomes. Some women are born with an XY chromosome and an otherwise typical female appearance — with some appearing to develop a penis and testes at puberty. All in all, as many as 1.7% of people have traits that could be considered “intersex,” a term that describes people born with sex traits that challenge the concept of a human sex binary.
Furthermore, many of the attributes that we think of as “male” and “female” can be changed through the administration of hormones, surgery, and the recognition of the government, which recognizes three categories of gender, a concept that is linked but not interchangeable with sex. Many bills restricting gender-affirming care for trans people contain specific carve outs for procedures to alter intersex bodies to conform with the binary.
It’s possible that in the future, more of the traits that make up human sex will be changeable. New techniques are giving scientists the ability to change chromosomes. Uterine transplants have already been performed successfully, including in cisgender women with surgically constructed vaginas. While it may be appealing to reduce the differences to humans to “basic biology,” reality is messy and complicated.