The operative constitutional text is section 1 of the Fourteenth Amendment, which was ratified in 1868. The relevant passages read as follows:
No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.The U.S. Supreme Court first applied this standard to marriage in Loving v. Virginia (1967), where it struck down a Virginia law banning interracial marriage. As Chief Justice Earl Warren wrote for the majority:
The freedom to marry has long been recognized as one of the vital personal rights essential to the orderly pursuit of happiness by free men ...While the U.S. Supreme Court has not yet ruled on same-sex marriage, it is unlikely that it would overturn the foundational premise that marriage is a civil right. Lower courts, even when relying on disparate state-level constitutional language, have consistently acknowledged the right to marry. Legal arguments for excepting same-sex marriage from the definition of marriage as a civil right have rested, instead, on the argument that the state has a compelling interest in restricting same-sex marriage that justifies limiting the right to marry (an argument that was also used to justify restrictions on interracial marriage), and/or that laws permitting civil unions provide a substantially equivalent standard to marriage that satisfies equal protection standards.
To deny this fundamental freedom on so unsupportable a basis as the racial classifications embodied in these statutes, classifications so directly subversive of the principle of equality at the heart of the Fourteenth Amendment, is surely to deprive all the State's citizens of liberty without due process of law. The Fourteenth Amendment requires that the freedom of choice to marry not be restricted by invidious racial discriminations. Under our Constitution, the freedom to marry, or not marry, a person of another race resides with the individual and cannot be infringed by the State.