It's recently come to light (at least in my little world) just how Transphobic Radical Feminists (as opposed to...other Radical Feminists...like me!) justify their exclusionary attitude toward transwomen, and basically what the Womyn Born Womyn "thing" really means.
They're falling back on biology, and thus a form of the very Essentialism the Radical Feminism I know has spent so much time deconstructing!
That is; a male produces smaller gametes (sperm) and a female produces larger gametes (eggs).
That's it. That's the entire biological definition of what it means to be male and female (and btw, some biologically male animals, like seahorses, for one, are the ones who carry children, and quite a few species don't remain "male" and "female" throughout their life! so, there is a lot in between what biology considers male and what we humans do).
In contrast, "man" and "woman" are social definitions. Each society, from insects, to fish, to mammals, each species has a society, which has gender roles. And many societies have multiple genders (again, from fish, to insects, to mammals, etc.). All of this has developed evolutionarily along with whatever keeps the species going. AKA what keeps them reproducing, and wanting to reproduce, and what keeps their society flourishing.
There are LOTS of really interesting examples of different strategies out there. From fish who have four different genders, with masculine females and feminine males and vice versa who pair up with whoever will work best with them. To our closest genetic relatives, who couldn't be more socially different, chimpanzees and bonobos. Chimps use a violent, patriarchal, loner sort of society to keep it together. Bonobos use extremely peaceful, and extremely sexual (tons of sex, between EVERYONE), matriarchal, communal society. And lots, lots, lots more...
My best guess is that this position is that since gender is a social construct, it should be broken down completely and done away with. That we will all be simply human, and be as masculine or feminine or (I assume) androgyne as we want to be. Or, it has been suggested there will be "no gender at all", which, personally, I think is unrealistic and rather silly. All animals have gender, it's not necessarily a bad thing.
I have also seen it said that Radical Feminism is only concerned with freeing females from oppression. Not women. Females. So, only those of us who naturally produce the larger gametes (this is why women with hysterectomies don't count as "non-women"). Of course, gods know what role this leaves for the intersex; the gametes they produce aren't necessarily telling of what their secondary sexual characteristics will be, so there's no easy place to put them in this construct.
Personally, I don't think this definition is a good one to go by for a social justice movement. The whole point of SOCIAL justice is to make society more just, and denying people's gender identities doesn't do that. We can affirm people's identities and still work against gender stereotyping that assaults all WOMEN. IMO we should be working to change that social definition, GENDER, "man" and "woman", not fighting against "male/female", why in the world does it matter to society (where we all live) what gametes we produce?? I don't see men ask women about the state of their eggs before raping them!
Far more important to me, what does it MEAN to be a "man"? What does it MEAN to be a "woman"? What does it MEAN to be neither, or both? What does it mean to be a masculine woman, or a feminine man? This is society, this is something we have control over, who "counts" as a man or a woman is up to us, the people who make up society. Who produces what size gametes doesn't tell us anything about a person, what they've gone through in life, who they are, it's a definition scientists use to differentiate between animals they're studying, even they say that's all it's good for (Evolutionary Rainbow, Joan Roughgarden).
But maybe gamete size is tangible. Maybe it's something solid that can unite us under one banner. "Woman" is so potentially malleable! It's maybe even confusing!
Life is confusing.
Someone tells me she is a woman, she lives as a woman, is treated by society as a woman, is in her own head and heart and soul a woman? She's a woman to me. I don't give a shit if her body produces eggs or sperm, that's not what matters. And same goes for a man.
Reducing complicated social problems to a biological black and white might be convenient, but it's wrong.