The Hungarian-born physician and psychoanalyst Sandor Rado (1890–1972), who practiced for most of his career in the United States, played a central role in shaping American psychoanalysts’ views toward homosexuality. Historians have pointed to Rado’s rejection of Freud’s notion of constitutional bisexuality as the key theoretical maneuver that both pathologized homosexuality and inspired an optimistic approach to its treatment. Yet scholarly analysis of the arguments that Rado made for his rejection of bisexuality is lacking. This article seeks to provide that analysis, by carefully reviewing and evaluating Rado’s arguments by the standards of his own day. Because one of Rado’s main arguments is that bisexuality is an outdated concept according to modern biology, I consider what contemporary biologists had to say on the topic. The work of behavioral endocrinologist Frank Beach (1911–1988) is important in this context and receives significant attention here. Rado ultimately distanced himself from Beach’s behavioral endocrinology, appealing instead to evolutionary discourse to buttress his claim that homosexuality is pathological. This tactic allowed him to refashion psychoanalysis into a moralistic discipline, one with closer ties to a medical school.