The institution of marriage has existed for millennia across civilizations. For the great majority of this time, marriage has been defined in terms of only one type of relationship – the union of one man and one woman. Marriage between a man and a woman was believed to be “essential to the very definition of that term and its role and function throughout the history of civilization.” This singular understanding of marriage has prevailed in the United States throughout our history. Indeed, at the time of the nation’s founding, marriage was understood to be a voluntary contract between a man and a woman. The framers drew heavily on legal scholars like William Blackstone, who regarded marriage between “husband and wife” as one of the “great relations in private life,” and philosophers like John Locke, who described marriage as “a voluntary compact between man and woman” centered on “its chief end, procreation” and the “nourishment and support” of children. To the drafters of the Constitution, this conception of marriage and family “was a given: its structure, its stability, roles, and values accepted by all.”