There is an enormous variation in sex and sexuality among animals. How has this diversity been explained by evolutionary theory? Evolutionary theory regarding sex differences has focused on males and their characteristics and despite theoretical development away from stereotypic notions of females and males, some gender bias remains, both in theory and empirical studies. Heteronormativity is abundant in evolutionary biology. Even though same-sex sexual behaviour has been described in more than one thousand species, the focus on reproduction, which is the basis for evolutionary theory, has long hindered the insight of how widespread it is. Biological arguments for what is natural are abounding in society, and this review of variation in biological sex challenges stereotypic notions of what femaleness and maleness mean. Finally, I discuss how we can aim for a non-normative evolutionary biology.