In the United States the question of whether couples of the same sex should be allowed to marry has roiled politics since at least 1993. In that year the Supreme Court of Hawaii heard a case in which the plaintiffs claimed that the state’s refusal to issue marriage licenses to same-sex couples abrogated those individuals’ rights to equal treatment under the law. The state, in turn, argued that it had a compelling interest in preventing same-sex marriage, as that practice would inherently damage the public good. The court found for the plaintiffs, basing its argument on the law’s absence of a clear definition of who might or might not participate in such a partnership. Soon after this finding, Hawaiian legislators added such a definition to the state constitution and thus made moot the issuing of marriage licenses to same-sex partners.

Many Americans felt that the Hawaii court decision represented a serious threat to social stability, and in 1996 the U.S. Congress enacted the Defense of Marriage Act. This legislation declared that same-sex marriages would not be recognized for federal purposes, such as the award of Social Security benefits normally afforded to a surviving spouse or employment-based benefits for the partners of federal employees. The act also restated existing law by providing that no U.S. state or territory was required to recognize marriages from elsewhere when it had strong policies to the contrary. Within a decade of the federal act’s passage, almost all the states had enacted laws or constitutional amendments declaring variously that marriage was legally defined as a heterosexual institution, that same-sex marriages from other states would not be recognized, or that same-sex marriage was contrary to the public policies of the state.

Nonetheless, some states moved toward the legal recognition of same-sex partnerships. In 1999 the Vermont Supreme Court declared that same-sex couples were entitled under the state constitution to the same legal rights as married heterosexual couples; shortly thereafter the state legislature enacted a law creating “civil unions,” which conferred all the rights and responsibilities of marriage but not the name. In 2003 California enacted a similar statute, calling the relationships “domestic partnerships.”

Also in 2003, the Massachusetts Supreme Court ruled that the denial of marriage licenses to same-sex couples violated the state constitution; the court gave the state six months to comply with its order to remedy the situation. The state soon began to issue marriage licenses for same-sex couples, but these were quickly challenged and their legal status over the long term remained uncertain. Officials in some smaller jurisdictions, notably San Francisco, joined the controversy in early 2004 by issuing marriage licenses in defiance of local prohibitions; these licenses were later found to be invalid. Subsequently, several other states and Washington, D.C., either established same-sex civil unions or adopted jurisdictionwide policies that accorded some spousal rights to same-sex couples.

Early in 2008 the Supreme Courts of two states—California and Connecticut—struck down state laws limiting marriage to the union between a man and a woman. Later in 2008 the voters in California passed a referendum, Proposition 8, that defined marriage as a union between a man and a woman. Proposition 8 subsequently was ruled unconstitutional in federal district court in August 2010 on the grounds that the prohibition of same-sex marriage violated the due process and equal protection clauses. Although advocates of the referendum quickly sought to contest the decision, it was upheld by a federal appeals court in February 2012. Referenda similar to California’s Proposition 8 were approved in Arizona and Florida in 2008 and in North Carolina in 2012.

In April 2009 the Iowa Supreme Court overturned a state law that barred gay marriage, and soon afterward the legislatures of Vermont, Maine, and New Hampshire legalized same-sex marriage—though in November 2009 Maine voters repealed the law. By 2011 Washington, D.C., and New York state had enacted similar legislation. In early 2012 bills allowing for same-sex marriage were passed by legislatures in Maryland and Washington state. Subsequent challenges to the laws made their implementation contingent on the results of ballot referenda, and in November voters in both states affirmed the laws. As the electorate in Maine simultaneously reversed its previous decision, the three states became the first in the country to approve same-sex marriage at the ballot box.

Broadly reflecting the community-benefit rhetoric noted above, many American legal scholars and same-sex marriage advocates developed arguments that the equal protection clause of the U.S. Constitution guaranteed the fundamental right to marry. Opposition arguments broadly reflected the procreative position and frequently invoked biblical exegeses or other religious doctrine to support claims that marriage, strictly defined, should be available only to heterosexual couples. Advocates of both perspectives cited various and conflicting sociological studies in defense of their claims. At the beginning of the 21st century a clear majority of the U.S. population opposed same-sex marriage, but by 2010 about half of the population supported legalization, and many of those opposed were open to the creation of legally recognized partnerships for same-sex couples.

In 2012, as public debate on the issue persisted, two significant events occurred at the federal level. Pres. Barack Obama, who during his initial years in office had endorsed only civil unions for same-sex couples, in May became the first sitting U.S. president to publicly support same-sex marriage. In December the U.S. Supreme Court announced that it would hear challenges to the Defense of Marriage Act and to Proposition 8. The following year the Court declared the act to be unconstitutional “as a deprivation of the equal liberty of persons that is protected by the Fifth Amendment” (United States v. Windsor), and it vacated the appeals court’s ruling regarding Proposition 8 on the grounds that the law’s defenders (a group of private citizens) lacked standing to appeal the district court’s order (Hollingsworth v. Perry).

Between December 2013 and August 2014, federal judges in 14 states overturned state bans of same-sex marriage. In all but two of those states, the rulings were stayed, although some of the states briefly performed same-sex marriages prior to their suspension. U.S. Attorney General Eric Holder announced that those marriages would be recognized by the federal government, and in February 2014 he introduced a Department of Justice policy to grant equal protection and treatment to all lawful marriages in the United States. In October the U.S. Supreme Court declined to review appeals of federal court decisions in five states, which effectively made same-sex marriage legal in those jurisdictions. As an indirect consequence, same-sex marriage was soon legalized by U.S. district courts in several additional states. By the end of 2014, the number of states where such marriages were legal had reached 35—more than twice as many as at the beginning of the year.

In January 2015 the Supreme Court agreed to review a November 2014 decision of the Court of Appeals of the Sixth Circuit that had upheld state laws and constitutional amendments banning same-sex marriage or the recognition of same-sex marriages performed in other jurisdictions. In June, in Obergefell v. Hodges, the Court reversed both of the Sixth Circuit’s holdings, thereby legalizing same-sex marriage in all 50 states.

In 2022 Pres. Joe Biden signed into law the federal Respect for Marriage Act, which formally repealed the Defense of Marriage Act, defined marriage as a legal union between two individuals, and required states to recognize same-sex marriages duly performed in other jurisdictions. Notably, the Respect for Marriage Act did not prohibit states from banning same-sex marriages, and it explicitly exempted religious institutions from the obligation to recognize such unions.