In the political discourse regarding gender identity, the concept of biological sex has been weaponised by gender critical commentators to oppose gender affirmation for trans people. Recently, these commentators have appealed to an essentialist model of sex based on anisogamy, or relative gamete size, to argue that one’s sex is an immutable characteristic. I argue that the gender critical argument is unsound. The diverse purposes of sex classification and the complex variability of people’s sexual characteristics show that an essentialist model is untenable. I then consider how a more adequate theoretical framework from the philosophy of biology can accommodate this complexity and capture how sex is classified in relevant contexts. Further implications of the framework are explored which concern the vagueness, polysemy, and mutability of sex. These undercut the gender critical argument and show that the appeal to biological sex fails to undermine gender affirmation for trans people.