Ever since the emergence of the religious right as a political force in the late 1970s, scholars and commentators have sought to explain its origins, often by depicting it as a reaction to the sexual rebellion and social movements of the preceding decade. But the true origins of our political and religious divides lie in sharp disagreements that emerged among American Christians a century ago. In the 1920s, after women gained the right to vote nationwide, a longstanding religious consensus about sexual morality began to fray irreparably. The slow but steady unraveling of that consensus in the decades that followed—over such issues as birth control, obscenity, interracial relationships, female chastity, sex education, abortion, sexual harassment, and LGBTQ rights—has transformed America's broader culture and public life, dividing our politics and pushing sex to the center of our public debate. This lecture describes how all of this unfolded and analyzes the pervasive fears driving our sex-obsessed politics.