We propose an externalist understanding of sex that builds upon extended and distributed approaches to cognition, and contributes to building a more just, diversity-sensitive society. Current sex categorization practices according to the female/male dichotomy are not only inaccurate and incoherent, but they also ground moral and political pressures that harm and oppress people. We argue that a new understanding of sex is due, an understanding that would acknowledge the variability and, most important, the flexibility of sex properties, as well as (...) the moral and political meaning of sex categorization. We propose an externalist account of sex, elaborating on extended and distributed approaches to cognition that capitalize on the natural capacity of organisms to couple with environmental resources. We introduce the notion of extended sex, and argue that properties relevant for sex categorization are neither exclusively internal to the individual skin, nor fixed. Finally, we spell out the potential of extended sex to support an active defense of diversity and an intervention against sex-based discrimination. (shrink)