With sustained challenges to European rule in African and Asian colonies in the early 20th century, sexual prescriptions by class, race and gender became increasingly central to the politics of rule and subject to new forms of scrutiny by colonial states. Focusing on the Netherlands Indies and French Indochina, but drawing on other contexts, this article examines how the very categories of “colonizer” and “colonized” were increasingly secured through forms of sexual control which defined the common political interests of European colonials and the cultural investments by which they identified themselves. The metropolitan and colonial discourses on health, “racial degeneracy,” and social reform from this period reveal how sexual sanctions demarcated positions of power by enforcing middle‐class conventions of respectability and thus the personal and public boundaries of race.[sexuality, race‐thinking, hygiene, colonial cultures, Southeast Asia]