Our contemporary debates about the nature of sex, marriage, and family life are not new. A half millennium ago, the Protestant Reformation set off a comparably tumultuous sexual revolution that bitterly divided the Catholic and Protestant worlds. Over the next cen- tury, jurists and theologians used various natural law theories to de- velop a common foundation for Western family law. In this Essay, I sample the writings of Dutch jurist Hugo Grotius (1583–1645) and English jurist John Selden (1584–1654)—-two leading Protestant nat- ural law theorists whose seminal writings helped to shape the Conti- nental civil law and the Anglo-American common-law traditions re- spectively. These two scholarly giants knew and respected each other, but they differed in their approach to natural law and its applications to family law and other legal questions. Grotius based his theory of natural law on rational self-evidence—-the rational inferences that can be drawn from human intuition and inclinations, common experienc- es and customs, and the nature of human sexuality and interaction. Selden based his theory of natural law on primeval divine commands, whose principles and precepts were worked out by great legal tradi- tions with enlightened leadership, most notably by the Jewish tradi- tion. Despite these different starting points and accents, both Grotius and Selden embraced a good number of traditional teachings on sex, marriage, and family life, albeit with nontraditional methods and rationales.