Sex segregated sport is increasingly untenable. As sports authorities and politicians enact flawed attempts to regulate who can and cannot participate in women’s and girls’ sports, the shaky foundations of the current system reveal themselves. Yet gender-integrated sports are perceived as unthinkable to many because they confront taken for granted beliefs about women’s inferiority to men. Sport is the only remaining institutional realm where sex segregation is permissible and, as such, it maintains and reproduces gender inequality. More than any other institution, sport showcases the cultural system of binary sex difference and makes it appear natural. Even people who, on all other fronts, believe in women’s equality to men, find themselves compelled by the argument that men are athletically superior to women. Gender expansive athletes, including trans, non-binary, and intersex athletes, threaten our social, economic, and political systems with disarray.