Sex is no more an immutable binary than is gender. There are intersex people who are born with non-binary genitalia, as I have already mentioned. There are people with hormonal anomalies. In fact, hormone levels vary wildly within the categories of cis male and cis female. Chromosomes, too, vary. If you thought “XX” and “XY” were the only two possible combinations, you have some serious googling to do. In addition to variations like XXY, XXYY, or X, sometimes cis people find out that they are genetically the “opposite” of what they though they were– that is, a ‘typical’ cis man can be XX, a ‘normal’ cis woman can be XY.
The fact is that the concept of binary sex is based on the fallacious idea that multiple sex characteristics are immutable and must always go together, when in fact many of them can be changed, many erased, and many appear independently in different combinations. “Female” in sex binary terms means having breasts, having a vagina, having a womb, not having a lot of body hair, having a high-pitched voice, having lots of estrogen, having a period, having XX chromosomes. “Male” means having a penis, not having breasts, producing sperm, having body hair, having a deep voice, having lots of testosterone, having XY chromosomes. Yet it is possible to isolate, alter, and remove many of these traits. Many of these traits do not always appear together, and before puberty and after menopause, many of them do not apply.