Here's a fantastic post from r/GC that explains bio-essentialism:

RESOURCES I'm seeing a lot of posts all over Reddit attempting to explain what gender critical feminists actually believe, and a lot of them are completely wrong. This is a post addressed to newcomers who are visiting this sub to figure out what gender critical feminism is all about.The first thing to get out of the way is this: no, we don't believe that you MUST have periods to be a woman. I've seen so many posts saying "JK Rowling is menopausal and doesn't have periods, so by her definition she wouldn't be a woman!" or "not every woman has a period, some women don't have uteruses!" She isn't saying that at all. Having a period is not a necessary condition for being a woman, but it is a sufficient condition. It's sufficient in the sense that if you experience menstruation then that automatically puts you in the woman category (or at the very least it puts you in the female category). But it's not it's not necessary that you must be capable of menstruation in order to be put into the woman category. In any case, what Rowling was upset about was female erasure. It is, quite frankly, deeply disturbing and insulting for many women to be called 'menstruators' or 'period havers'. This , we feel, reduces us to a biological function. The 'bloody good period' charity once referred to women as 'bleeders', and not only does this not make any sense (men can bleed too!) it is also degrading. 'Bleeders' sounds like a dehumanising slur. Notice there's a pattern here - when have you ever heard males been referred to as 'ejaculators' or 'sperm producers'? I'm going to try to debunk two other claims I've seen floating around that are just bollocks. The first is that gender critical feminists conflate sex with gender. The second is that we are biological essentialists. Both of these claims are false, and both particularly sting because a lot of gender critical feminists of the first and second wave of feminism actually railed against these concepts in the first place. So, back to basics. Sex is biological. Sex refers to the two reproductive roles - male and female. Males produce small mobile gametes, sperm. Females produce the large immobile gametes, eggs. Sex is not a social construct. It is found all across nature. Plants have sexes. Animals have sexes. Gender is a social construct, and is not the same thing as sex. This was recognised by Mary Wollstonecraft in the 18th century, but was really emphasised by the second wave. Gender is social norms of femininity and masculinity. Gender are roles, behaviours, stereotypes, and expectations placed on human beings according to their sex. Male humans are supposed to live up to masculine gender expectations. Female humans are supposed to live up to the feminine gender expectations. Boys are supposed to like violent sports, getting angry, not crying, not eating salad, the colour blue, being dominant and in control, being logical. Women are supposed to like kittens, being sensitive and empathetic, watching their weight, pink, babies, being a homemaker. Women are human females who have the feminine gender imposed on them. And men are human males who have the masculine gender imposed on them. And gender also operates as a hierarchy. Femininity is inferior to masculinity. Feminists pointed out there was nothing natural about gender. Women do not naturally want to be nothing more than baby makers, who cry and like pink glitter, who only care about adornment and home making. This is all a social construct. Gender operates as a constrictive trap, forcing women into one 'natural' lifestyle.A big project of the second wave was to point out the artificiality of gender roles, and to break women out of them. There is no one way to look like a woman. Women can wear makeup, have short hair, wear whatever they like. There should be no such thing as masculine and feminine interests - why shouldn't a man enjoy knitting? Why does that have to mean he's somehow less of a man, or a sissy? And why shouldn't a woman enjoy car racing? Why are we so obsessed with gendering everything, from clothing, to food, to hobbies, to pets, to cars? Second wave feminists established that just because someone is female doesn't mean they are, or should be, girly. Women do not naturally behave, or think, in a particular way. Gender critical feminists have always held fast to this idea that there is no necessary connection between sex and gender roles. If I tell you I have blue eyes, can you infer anything at all about what my tastes are from that information alone? What is my career? My hobbies? My interests? My behaviour? My aspirations? My overall appearance? No -of course not. That is because we don't imbue having blue eyes with any significance. Blue eyes has no connection to your personality or your cognitive capacities. Well, that's what gender critical feminists think about sex. If I tell you a person is female, that doesn't necessarily tell you anything about her hobbies or interests. At most it tells you she's a human who is very likely to experience the incidents of being female - she is likely to be capable of menstruating. She is likely to be capable of getting pregnant. Beyond those basic facts of biology, though, females are people and can have a wide range of behaviours and appearances. There is no 'right' way to be a woman. Literally the only precondition is that you be female. That's it. Female biology is different from male bioloogy, and due to us living in a sexist society is often pathologised and overlooked. For example, menstruation has been coded as 'unclean' and impure - gender stereotypes are interposed on top of natural biological functions. So what Rowling was trying to emphasise was that for women our sex is deeply important to us. It's a core part of our experience going through the world. We are stigmatised for having periods, stigmatised for going on contraception, stigmatised for abortions, stigmatised for giving birth (too many or too few babies). And so there are important political concerns that only apply to those with female bodies. Also, female bodies are often a locus of state control. The state will try to control women by controlling their bodies (see abortion legislation, forced sterilisation,) What that means is even if you accept that trans women are 'women' (whatever women means) there must still be room for a distinctive female rights movement. Some dude on the Reddit front page was given hundreds of awards for blithely saying "no one is saying sex isn't real - we are just saying that sex isn't important." With no respect, dude, it's just not your place to say that. Women are pointing out that sex IS important to them - women are dying because they are taking drugs that have only been tested on male bodies. Men can't handwave that away with claims that 'sex isn't important'. WE are telling YOU that it is. It's important. It's not bigoted for women to campaign for their sex based rights. Biological essentialism is related to all this. Biological essentialism is the view that sex roles are natural. BECAUSE I am female it is therefore natural for me to like babies, be more emotional, and to desire to go shopping. But as I already covered, this view was contested by second wave feminists. On average, women may enjoy makeup more than men. They may be more caring and nurturing, They. may be less into train and plane spotting. But none of this is natural. Women aren't born this way. There is no natural connection between women being female and women liking makeup. There's no biologically essential way for women to be. You may notice that people on Reddit just don't seem to really get what biological essentialism means. Biological essentialism has nothing to do with saying 'women have female bodies.' It's not 'essentialist' to say that women are female. That's just part of the definition of woman. Biological essentialism is the claim that because someone is a woman they have to behave or think in a particular way. In actual fact, many commenters will support trans theory using biological essentialism. One male commenter bravely attempted to define women - " a woman is someone who conforms to our societies expectations of femininity - they dress in a feminine way, think in a feminine way, have feminine interests and so on." This is pure essentialism. Gender critical feminists completely deny that there is any such thing as 'feminine' thoughts. What on earth is a 'feminine' way of thinking? If you say that women are more ditzy, creative, and less logical - isn't that just super sexist? And what's a 'feminine' interest? Are we really back to the bad old days of saying you are less of a woman if you enjoy dirt bike racing? Are you less of a man if you film makeup tutorials? On this definition I would simply fail to be a woman. I wear pants. I don't have typical feminine interests. I work in a male dominated career. I find this definition really sexist - it says that women have to think and act, and look a certain way in order to be women. they have to conform to femininity. But the definition that a woman is an adult human female is not biologically essentialist because all it tells you is that women have female bodies. That's it. Women can look and act in any way they like. To recap and clarify - gender critical feminists are against biological essentialism. And they do not think that sex and gender are the same thing!