" You can't assert that anatomy isn't destiny (as most, though of course not all, third wave feminists do) and then complain when someone asserts a gender identity that challenges anatomical gender norms. "
Qwerty - I think in a way that gets to the basis of the problem with third wave feminism. Yes, anatomy isn't destiny, but for second wave feminists, it is the material condition against which to make sense of our oppression. When you look at what is done to women world wide - restrictions on contraception and abortion, sex-selective abortion, female infanticide, female genital mutilation, restriction of women's access to education, jobs, and the public political sphere - this isn't happening for an arbitrary reason. It's not like an imaginary dystopian society which arbitrarily decides to discriminate against, I dunno, people with ginger hair. Two things make sense of systematic and widespread, cross-cultural, cross-historical oppression of women, and that is men wanting to control women's reproductive labour (because women are the sex that have children, not men) and "because they can" - because the obvious, inescapable sexually dimorphic characteristic in humans is that men are on average, and with quite a large d value (see thread on brain sex) bigger and stronger than women.
But the crucial thing is that none of this makes it ethically justified to oppress women - that's the point, to my mind, of feminism. That yes, there are biological differences, but no, these do not justify treating women as less than entirely human - denying them the vote, paying them less for the same work, not paying them at all for a lot of their work (because "caring" isn't important), denying them the right to own property, denying them the right to bodily autonomy (saying no to sexual partners, saying no to unwanted pregnancies).
The claim that biology is not destiny in the sense that biological sex has nothing to do with personhood is not the same as saying biological sex is irrelevant to understanding the world and to making sense of why women are oppressed and denied personhood in a lot of cultures and political systems.