This essay looks at one of the perennial sites of conflict over both orthodoxy and orthopraxy: the regulation of sexual morals, which brings both issues to a sharp point. The essay considers how the notorious scandal around the bigamy of the Lutheran prince Philipp of Hesse in 1539–40 placed emerging Lutheran understandings of orthodoxy under excruciating pressure. In this new world, where long-settled doctrines were suddenly open to negotiation, it was not entirely unreasonable of Philipp and his supporters to appeal to the ambiguous biblical evidence and to question whether Christianity necessarily entailed monogamy. At the moment of the crisis, the blunt realities of power politics pushed Martin Luther in the same direction. Yet, over the decades that followed, those same blunt realities meant that Lutheranism had to be seen to reaffirm traditional sexual orthodoxies. Even if theologians’ conservative instincts had not tended this way in any case, the risk of reputational damage from not disavowing Philipp’s adventures was intolerable. The result was the reverse-engineering of a distinctive theological method: since sola scriptura could not be relied on to provide a sufficiently unambiguous answer to the question of polygamy, Lutherans in the so-called age of orthodoxy turned to natural law instead.