People think that biological sex is this straightforward thing where if you have two X chromosomes you're female and if you have an X chromosome and a Y chromosome you're male and this is extremely not how things work.

The most obvious cases: you can have a single X chromosome. You can have three X chromosomes. You can have two X chromosomes and a Y chromosome. You can have an X chromosome and two Y chromosomes. Basically the only thing that doesn't work is having zero X chromosomes.

Ok so how about we say "If you have a Y chromosome you're male and if you don't you're female"? Well, no, that also isn't how things work. There's a gene on the Y chromosome called SRY. If that ends up on an X chromosome, you have XX people who are usually assigned male at birth.

Ok, so how about we say "If you have a copy of SRY you're male and if you don't you're female"? Well, no. You can have a copy of SRY, but if you don't have a copy of NR5A1 you're still going to be born with a uterus and not a penis.

And at this point it starts getting complicated. There's a whole cascade of genes that are involved in which genitals you end up with. We literally don't have a complete understanding of everything that's involved.

But we're still largely focusing on SRY, which is only one of the genes that the Y chromosome carries. What do the others on the Y chromosome do? We largely don't know! They're presumably tied to "maleness" in some way, otherwise they wouldn't be on the Y chromosome.

You can be born with a penis, present as male your entire life and still be missing most genes that are on the Y chromosome. Does that make you more or less male? (spoiler: no)

There's no single biological trait we can point at and say "male". Some people will argue in terms of "Female = generates larger gamete, male = generates smaller gamete", which is strange because you don't generate gametes when you're born.

(As an aside, when you point out to people using this definition that there are plenty of people who never produce gametes at all, they'll start framing them as "defective males" or "defective females", which really tells you something about their empathy levels)

Remember how science works: if there's a *single* counterexample to your hypothesis, your hypothesis is wrong. Nobody has presented a biological definition of sex that has no counterexamples. There is no real scientific definition of sex.

But even so, using biology to define what sex is is an arbitrary choice. People categorised each other before we had any understanding of biology. Splitting people into "male" and "female" isn't socially or historically universal.

So sure, you can say the meaning of male is "was born with a penis", but that's not a scientific definition, and nor is it a social definition that everyone agrees with. It's an arbitrary choice, not inherent truth.

tl;dr: the useful definition of "male" is "someone who says they're male". The useful definition of "female" is "someone who says they're female". There is no accurate scientific definition. Policy built on the idea that science defines sex is not policy that defends science.

Basically: if you don't want trans people to get healthcare, just say so. Don't pretend the science made you do it.

(So why do biologists use the terms 'male' and 'female'? The simple answer is because biology is so complicated there's no way for humans to understand it usefully unless we put stuff in boxes, and we know that those boxes are arbitrary and inaccurate)