Same-sex marriage, also known as gay marriage, is the marriage of two people of the same legal sex. As of 2024, marriage between same-sex couples is legally performed and recognized in 35 countries, with a total population of 1.3 billion people (17% of the world's population). The most recent country to legalise same-sex marriage is Estonia. Greece is expected to pass same-sex marriage in February 2024.
Adoption rights are not necessarily covered, though most states with same-sex marriage allow those couples to jointly adopt as other married couples can. In contrast, 35 countries (as of 2023) have definitions of marriage in their constitutions that prevent marriage between couples of the same sex, most enacted in recent decades as a preventative measure. Some other countries have constitutionally mandated Islamic law, which is generally interpreted as prohibiting marriage between same-sex couples. In six of the former and most of the latter, homosexuality itself is criminalized. There are records of marriage between men dating back to the first century. It is legally recognized in a large majority of the world's developed democracies; notable exceptions are Italy, Japan, South Korea, Greece and the Czech Republic. It is not yet recognized in any of the world's Islamic polities. Some countries, such as China and Russia, restrict advocacy for same-sex marriage.
The first same-sex couple to be married legally in modern times were Michael McConnell and Jack Baker in 1971 in the United States; they were married in the county of Blue Earth County, Minnesota. The first law providing for marriage equality between same-sex and opposite-sex couples was passed in the continental Netherlands in 2000 and took effect on 1 April 2001. The application of marriage law equally to same-sex and opposite-sex couples has varied by jurisdiction, and has come about through legislative change to marriage law, court rulings based on constitutional guarantees of equality, recognition that marriage of same-sex couples is allowed by existing marriage law, and by direct popular vote, such as through referendums and initiatives. The most prominent supporters of same-sex marriage are the world's major medical and scientific communities, along with human rights and civil rights organizations, while its most prominent opponents are religious fundamentalist groups. Polls consistently show continually rising support for the recognition of same-sex marriage in all developed democracies and in many developing countries.
Scientific studies show that the financial, psychological, and physical well-being of gay people are enhanced by marriage, and that the children of same-sex parents benefit from being raised by married same-sex couples within a marital union that is recognized by law and supported by societal institutions. Social science research indicates that the exclusion of same-sex couples from marriage stigmatizes and invites public discrimination against gay and lesbian people, with research repudiating the notion that either civilization or viable social orders depend upon restricting marriage to heterosexuals. Same-sex marriage can provide those in committed same-sex relationships with relevant government services and make financial demands on them comparable to that required of those in opposite-sex marriages, and also gives them legal protections such as inheritance and hospital visitation rights. Opposition is based on claims such as that homosexuality is unnatural and abnormal, that the recognition of same-sex unions will promote homosexuality in society, and that children are better off when raised by opposite-sex couples. These claims are refuted by scientific studies, which show that homosexuality is a natural and normal variation in human sexuality, that sexual orientation is not a choice, and that children of same-sex couples fare just as well as the children of opposite-sex couples.