Under the Constitution as originally understood, jurisdiction over domestic relations outside federal enclaves and federal territories was reserved to the states.
State laws dealing with domestic relations are, however, subject to the mandates of the Fourteenth Amendment. That provision requires states to honor “due process of law” and to grant all citizens “equal protection of the laws.” (The Supreme Court has added that the Fifth Amendment imposes a similar equal protection standard on the federal government; a more credible reading of the Constitution would derive some federal equal protection obligations from the “proper” component of the Necessary and Proper Clause.)
Modern Supreme Court Due Process cases recognize a constitutional right to make free sexual and co-habitation choices, both heterosexual and homosexual. People also have the right to obtain religious sanction for unions not recognized by state civil marriage laws. But there is no general federal constitutional right to the government benefits bestowed by state civil marriage laws; indeed, the states are not required to adopt civil marriage laws at all. Some commentators argue we would be better off if they didn’t, and formal recognition of marriage were left to the private sector.
It is important to understand what civil marriage laws are. They are enactments authorizing the grant of special state benefits for certain kinds of unions. In other words, they give to people in qualifying relationships what the Framers called “privileges and immunities”—an 18th century legal phrase that refers to benefits bestowed by government on some people to the exclusion of others. American governments traditionally have conceded the “privileges and immunities” of civil marriage only to a social union complying with certain exacting requirements. With some variations, state laws traditionally require that the union be (1) of a man and a woman, (2) who undergo certain procedures in advance, (3) obtain a valid license, (4) have consented, (5) are above a certain age, (6) are not married to any one else, (7) are not too closely related to each other, and (8) meet certain other requirements of ceremony and/or cohabitation. States traditionally have excluded from special benefits all other groupings—including, but not limited to, same-sex marriages, polygamous marriages, polyandric marriages, other plural clusters, designated intra-family unions (e.g., brother/sister and uncle/niece), and unions that are unlicensed or that otherwise fail to meet the states’ rules.
Now we come to the big question: Does a state’s decision to grant benefits to just one sort of union violate the equal protection doctrine? A grant of special privileges to one group while excluding others does violate that doctrine unless the state can point to legitimate public reasons for its decision. How strong the reasons have to be depends on the kind of case. For better or worse, the Supreme Court is very tolerant of government discrimination among economic classes. In social-issue cases, on the other hand, the Court sets more exacting standards.
It is clear that for constitutional purposes civil marriage laws that include one man/one woman unions do meet those exacting standards. This is because of the overwhelming evidence of social benefit deriving from such unions. This evidence arises both from formal empirical studies and from practical experience gathered, quite literally, over millennia.
What about extending the “privileges and immunities” of civil marriage to other groupings? That’s a much tougher case to make because, with the arguable exception of polygamous marriage, the supporting evidence is so much weaker. Particularly in the case of same-sex marriage, the evidence of social benefit is spotty and highly politicized. Under Supreme Court Equal Protection jurisprudence, it is not strong enough to require states to recognize such unions.
Thus, under existing Supreme Court Equal Protection doctrine, the real issue is not whether the evidence is sufficient to compel states to recognize same-sex marriage. The real issue is whether that evidence is sufficient to justify states granting to same-sex couples “privileges and immunities” that most other groupings—such as polygamous and polyandrous unions—do not receive.
My own view, for what it is worth, is that such questions are best left to the individual states to resolve. That doesn’t address the special problems arising under the federal Defense of Marriage Act, but it’s a good general principle.