> Biological sex isn't binary

For the vast, vast majority of people, it is. Which is what makes it a useful metric.

Most people may seem to fall tolerably close to one of two stereotypes, but that doesn't make biological sex binary, not does it make it sensible to ignore the fact that it is not in the case of those people who do not, and, surreptitiously enough, there's a significant overlay between people whose experienced gender doesn't match their socially ascribed gender and people whose biological sex-related traits other than external genitalia don't line up with external genitalia in the most common way.

Male and female are not "stereotypes". Biological sex is the most binary thing since zero and one.

> Biological sex is the most binary thing since zero and one.

It is really not, though for most of human history a reasonably informed person could justifiably believe that; but even that hasn't been true for decades, at least.

At that point the female/male distinction loses its meaning altogether. If anyone can pick their gender, what makes someone masculine and what feminine? Nothing. There's no difference between genders, so lose gender-specific words.

I'm not against transgender, I'm just arguing on technical grounds. I don't see value in using gender-specific words if people can pick their gender, but I do see value in using them purely to distinguish between people by what is between their legs.

> If anyone can pick their gender, what makes someone masculine and what feminine?

I don't think this really follows. There are still traditionally masculine and feminine traits, this is unchanged by people self-identifying as a gender other than the one they were born with.

Species isn't a fixed concept. If you think what's expressed as someone's phenotype is more important that the person themselves, don't be surprised if refusing to refer to someone as a dragon, and subjecting them to objectification as a human being, produces negative responses.

What's between our legs matters most.

If I am biologically male but identify as female, then guess what? I'm never going to become pregnant and give birth, no matter what mutilation of the English language I use to refer to myself.

Wait, are you arguing external genitalia matter most, or are arguing that having (or not having) fully functional female internal reproductive organs matters most? Because you first sentence seems to say the former and your other paragraph the latter. While there is obviously a correlation between those things, it's not an absolute one.

>Biological sex isn't binary

It most certainly is. XY and XX are two things, therefore binary. Anything else is a defect in the individual's chromosomes.

XX and XY have a strong correlation with sex traits, but not a 1:1 correspondence. (And neither those chromosome configurations nor other biological sex traits map 1:1 to external genitalia, which is Italy the basis of socially ascribed gender.)

So the fact that XX and XY configurations exist and other configurations are considered "defects" doesn't make biological sex a binary trait that correspondence to the manner in which gender is usually digitally ascribed and for which people are labeled "transgender" when their gender identity differs from that social judgement.