The role of sexuality has generally been overlooked in sociological and economic research, partly due to the lack of representative data. We present a new theory of sexuality as the underlying source of patriarchy in the Twenty-First century, drawing on nationally representative surveys, qualitative and case study evidence on sex differences in sexuality. Lerner's  explanation for the creation of patriarchy is updated to identify sexuality rather than fertility as the driving force for male attempts to maintain male dominance in private and public life today, because recreational sexuality is now more important than reproductive sexuality. Sex surveys across the world carried out in 1990-2010 reveal a persistent sex differential in sexual interest and motivation, resulting in a sexual deficit among men. Strategic case studies of two sexual spot markets (internet dating and commercial sex) illuminate the continuing patriarchal ideological response today to women who exploit the male sexual deficit. The male sexual deficit helps explain the stigmatization of women selling sexual services and entertainments, sexual harassment, sexual violence, and rape - even in Western liberal democracies. The male sexual deficit is increased by women's access to higher education and independent incomes, leading some women to withdraw from sexual and marriage markets. For different reasons, patriarchal men and feminist women often deny the significance of sexuality in sexual politics debates.