sex != gender

Biological sex determines the bits you have in your pants - gender on the other hand is the social construct we humans have created, on top of that, to assign social roles and expectations to individuals.

The main issue TERFS (or FARTS) have, is that they argue the only thing that matters is what bits you have in your trousers. Which completely ignores the reality of people everywhere that are societally expected to behave in certain ways because it aligns with what society has deemed "manly" or "girly". They argue that by wanting rights for trans people, that means, we, as a society, end up entrenching the gender norms feminists have fought so hard to dismantle.

However, that fully relies on the idea that gender has already been abolished and everyone presenting the way they truly feel is just "men" co-opting the feminist movement for their own "deviant" benefit.

Which

A. Completely misses the point about equality and solidarity (why does it matter if AMABs present as women if we all have equal rights?)

B. Disregards the reality of transmasc, transfemmes, enby and anyone else that just wants to live life in a form they feel comfortable with (feminine women and masculine men are just as valid as the opposite)

C. Absolutely dismisses the experience of transmasc individuals as "confused girls" - which is not only bigoted but extremely sexist, it implies that AFABs have no agency and are fully controlled by society - "you see they're not smart enough to understand social constructs and how their lives fit into them - no! they're just trying to pass as male so they get the patriarchal benefits the current system provides!" - again completely missing the point of equality.

You may think these are strawmen but if you're familiar with JKs rethoric you'll see these are genuine beliefs she holds.

This way, the "Trans Movement" (ie. people presenting and behaving in ways not directly assigned to their birth sex) becomes a hill to die on.

Not because of the purported "safety of girls" in bathrooms, sports and prisons (which JK will gladly demonize in her own fictional worlds of trans criminals and rapists (spoilers - people of any sex or gender expression can be awful human beings - saying they're the reason sex crimes occur just dismisses the reality of abuse perpetuated by hurtful people))

But because it's an ideology of absolute societal constructs (how can a man pretend to be a woman if the only thing that makes a woman is her genitalia?)

And look, I too am a gender abolitionist - if we lived in a genderless society that didn't have gender reveal parties or gendered shoes or clothes or interests or literally anything that divides people into binary groups - I'd be on that shit - but that's not what folk like JK are fighting for.

They see sex as this immutable quality that not only determines your reproductive organs - but how society should perceive you. You have a penis - you are a man. Oh, you don't like being perceived as a man and you feel you relate to women in a way that other men don't? Well, too fucking bad, in the man-bin you go. Rooster between your legs says you're not allowed to sit in a female-only cubicle - get the fuck out.

That's why this type of thinking is harmful, the goal isn't to see who gets the "most rights", the goal is for everyone to get the same amount of rights, always - so it doesn't matter if you call yourself a man, a woman or any other label you may choose - what matters is that if you give out love and respect you should receive the same in return.

Edit: Sorry for the massive essay, but assumed you might be interested in the context around sex and gender

No, they're telling you that gender is not sex.

Of course biological sex is real. It's just a lot more complicated than 'male' and 'female' because biology is not that simple. So yes, there are XY and XX humans and they can generally, but not universally, reproduce with each other. But that's not all there is. For instance XXY and XYY are both possible, although often come with a host of other genetic problems (but not always). There are also people born with both types of genitalia, sometimes functional and sometimes not. On top of that, there are conditions like Swyer Syndrome, where someone with XY chromosomes has female genitalia and maybe even a functional female reproductive system.

If trans people are telling you that biological sex is not male and female, that is what they mean.

Trans people aside, there are people who are purely biologically not XY-male or XX-female. You can have X, Y, YY, XYY, XXY, and XXYY. And more, but the further you get from a pair, the lower the viability. Not to mention things like hormone insensitivity, where you may not develop primary or secondary sexual characteristics, or having sensitivity and developing too much in a certain way.

It's all very complicated, and honestly when it doesn't affect me I don't worry about it. Let people live their lives the way they want in peace and everyone will be much happier.