Feminist theory needs a constructivist account of biological sex for at least two reasons. The first is that as long as female and male are the only two sexes that are taken for granted, being cisgender, heterosexual, and preferably a parent will be the norm, and being intersexed, transgender, bi- or homosexual, infertile or voluntarily childless will be deemed failure. The second is the fact that, usually, sex and gender come together in the way that is expected, i.e. the fact that most females are women and most males are men needs to be explained. This paper provides a constructivist theory of sex, which is that the sex categories depend on norms of reproduction. I argue that, because the sex categories are defined according to the two functions or causal roles in reproduction, and biological functionis a teleological concept involving purposes, goals, and values, female and male are normative categories. As there are no norms or values in nature, normative categories are social constructions; hence, female and male are not natural but social categories. Once we understand that biological normativity is social, biological norms of heterosexuality, fertility, and so on are no longer incontestable. In addition, as many gender norms also concern reproduction - socially mediated reproduction - this simple theory of sex explains the common confluence of sex and gender.