The process of social differentiation, or the process of creating boxes in which we can place other people and in which we can place ourselves, is key to the existence and persistence of social inequality. The focus of this article is on the construction and maintenance of boxes and boundaries with respect to sex, gender and sexuality. We take the existence of these boxes and boundaries for granted, organizing our lives around them in a variety of ways. Exceptions to them call our categorizations and the decisions we make based upon them into question. Particularly interesting in this context are intersexuality, transgender and bisexuality. Intersexuality, transgender and bisexuality have in common the fact that they challenge our easy reliance on categories and the boundaries between those categories. Our responses to, treatment of and understanding of these exceptions provides striking insights into our system of boxes and boundaries and, correspondingly, to sex-, gender- and sexuality-based inequalities.