Every debate about sex or gender rolls out some high school biology somewhere along the line. There are two sex chromosomes, X and Y, and they combine in specific ways to produce two (and only two) different outcomes: XY = male, XX = female. Guess what?
It’s more complicated than that.
If you dig into the actual biology, not only is it possible to have other combinations than plain ol’ XY and XX (like XXX and XXY), but even when those chromosomes are present and correct, the finished product doesn’t necessarily come out with the expected genitalia of the expected size, location, etc. Instead, as with many things influenced by more than one gene, the real process of sexual development leads to much more of a continuum — and it’s not entirely clear how many people actually have some of that variance in a way that might never be observable. Consider the fate of XXY males (lots of variation here, but can include small male-typical genitalia, large breasts, broader hips than is typical for XY males, sterility…), XY females (female genitalia, will not experience puberty, sterile) and XX males (may possess male-typical or female-typical genitalia, or both, and highly variable degrees of other masculine development characteristics).
How common is some degree of variance in sex characteristics? It’s thought to be about 2% of the population — 2 in 100 births. That’s probably an underestimate, as many people would never realise there was any variance, just putting it down to variance in other characteristics or never thinking anything of it at all.
We have the male and female designations because much of the time it seems to be completely clear, and it can be useful medically, but all kinds of complexities lie behind the biology. Whatever argument you want to make about sex or gender, it’s probably best to leave biology out of it unless you’ve actually done the research. The obvious conclusions based on the world you think you see around you are just wrong.