Today, 32 countries — representing 15% of the world population — legally recognize same-sex couples in some form.

Also, beginning in the late 20th century, there has been a growing global movement to regard marriage as a fundamental human right to be extended to same-sex couples. These events are extraordinary given that during most of the 20th century, homosexuals were closeted — and the concept of same-sex marriage was inconceivable.

Marriage and family have been undergoing noteworthy transformation. Non-traditional arrangements, such as cohabitation, non-marital childbearing and inter-racial and inter-religious relationships have become more common and accepted, especially among Western nations. In contrast, acceptance of same-sex relationships continues to face considerable opposition, hostility and suppression.

Policies and laws regulating marriage — as well as sexual activity — vary widely among countries, ranging from permitting same-sex marriage to the imposition of the death penalty for same-sex sexual activity. Same-sex sexual acts by consenting adults acting in private are legal in some 100 countries, or about 60% of the world’s population. By contrast, in more than 70 countries — accounting for 40% of the world population — same-sex sexual acts by consenting adults acting in private are considered a crime, with some imposing fines, corporal punishment and imprisonment.

The legal recognition of same-sex couples did not occur until 1989, when Denmark became the first country to legally recognize them as officially registered partnerships. Today, 32 countries — representing 15% of the world population — legally recognize same-sex couples as domestic partnerships, civil unions or registered/unregistered cohabitation with benefits similar to those received by married opposite-sex couples.

In 2001, 12 years after Denmark legally recognized same-sex partnerships, the Netherlands became the first country to permit same-sex couples to marry legally. As of mid-year 2011, ten countries (Argentina, Belgium, Canada, Iceland, the Netherlands, Norway, Portugal, South Africa, Spain and Sweden), Mexico City and six U.S. states (Connecticut, Iowa, Massachusetts, New Hampshire, New York and Vermont) and Washington, D.C., have legalized SSM. (A seventh U.S. state, California, formerly recognized SSM until the passage of Proposition Eight in November 2008, which amended the California constitution to limit marriage to opposite-sex couples. The status of SSM in the state is currently being determined by the courts.) Together, these countries, cities and U.S. states have a combined population of 319 million, or 5% of the world population.

In more than 70 countries — accounting for 40% of the world population — same-sex sexual acts by consenting adults acting in private are considered a crime.

The legalization of same-sex marriage is also being considered by other countries. For example, a Brazilian state judge in Sao Paulo recently ruled that under the Brazilian constitution, a same-sex union could be converted to a same-sex marriage. Also, Nepal’s Supreme Court gave its consent to same-sex marriages, ordering the country's government to craft new laws to guarantee full rights to gays and lesbians, including the right to marriage. The legalization of same-sex marriage is also under discussion in Australia, Colombia, Finland, Luxembourg, the United Kingdom and Uruguay.

The levels of same-sex marriage are relatively low, typically accounting for 2-7% of all marriages contracted in a single year after the first year the practice is legalized. During the first year, and sometimes well into the second year after same-sex marriage is legalized, an initial spike in the number of SSM is often observed, followed by a tapering off to low levels. In addition, some areas may continue to have elevated levels because they function as “same-sex marriage magnets,” attracting relatively large numbers of non-resident same-sex couples wishing to marry.

By the end of 2009, the total cumulative number of same-sex marriages that had legally taken place worldwide was at least 100,000. However, as data for some countries and recent years for others were not readily available, this figure is no doubt considerably less than the actual total cumulative number of same-sex marriages.

The majority of the same-sex marriages in the three European nations that support the practice (i.e., Belgium, the Netherlands and Spain) were gay marriages — that is, between two males. However, in Norway and Sweden as well as in Connecticut, Iowa, Massachusetts and Vermont, most SSM were between two females. Possible reasons for the higher proportions of lesbian marriages in those countries and U.S. states include higher levels of skepticism about marriage among two males, the need for a two-income family among lesbian women, more lesbian couples rearing children and greater likelihood for lesbians to be sexually monogamous than gay couples.

The levels of same-sex marriage are relatively low, typically accounting for 2-7% of all marriages contracted in a single year.

Available trend data on age at marriage from Massachusetts indicate higher median ages at marriage for persons in gay marriages, followed by those in lesbian marriages and then opposite-sex marriage. However, these differences have been declining over time as the backlog of older same-sex couples wed.

Same-sex marriages are more likely to divorce than opposite-sex marriages. Also, data from Belgium, the Netherlands and Spain indicate that lesbian marriages are more likely to end in divorce than gay marriages. Also, among opposite-sex marriages, divorces are more likely to be initiated by women. Preliminary findings suggest that in both same-sex and opposite-sex marriages, women are more likely to divorce than men, suggesting that women experience more dissatisfaction than men with the institution of marriage.

Public opinion on same-sex marriage not only varies greatly across regions and countries, but is in flux. Opinion surveys in some Western countries point to increasing support for SSM, especially among younger people. In others, the publics are prepared to legally recognize same-sex couples as partnerships but not as marriages, because they fundamentally believe that marriage should remain as a union between a man and a woman.

Also, whereas the public debate in many countries centers on the legal recognition of same-sex unions and marriage, in other parts of the world the question concerns the acceptability of homosexuality itself. Strong majorities of the public polled in Africa and Asia do not view homosexuality as a socially acceptable way of life.

Twenty countries, primarily in Europe, recognize same-sex couples for immigration purposes. The U.S. government does not allow the naturalization of same-sex spouses or life-partners on the basis of direct family ties, nor does it recognize the legality of same-sex marriage.

By the end of 2009, the total cumulative number of same-sex marriages that had legally taken place worldwide was at least 100,000.

While six U.S. states permit same-sex couples to marry, 40 states have statutes prohibiting same-sex marriage. In addition, the Defense of Marriage Act (DOMA) bars federal recognition of same-sex marriage, allowing states to do the same, and defines marriage as a legal union between one man and one woman. While the Obama Administration considers DOMA unconstitutional and the U.S. Senate is considering a bill to replace it, the U.S. House of Representatives has reaffirmed DOMA. The constitutionality of DOMA is very likely headed to the U.S. Supreme Court.

At the international level, following several European nations’ recognition of SSM and in the face of strong objections and protests by many member states, the United Nations decided in 2004 to recognize the SSM of staff as well as legally recognized domestic same-sex partnerships and provide benefits and entitlements to eligible family members if SSM is legal in their country of nationality. Also more recently, a joint statement on behalf of 85 countries at the United Nations Human Rights Council called on all states to end violence, criminal sanctions and related human rights violations based on sexual orientation and gender identity.

Marriage and the family are undergoing fundamental transformations that are resulting in social and political stresses and tensions as well as legal challenges well beyond the borders of those countries and areas where SSM is legal. These consequences are increasingly influencing global norms, intergovernmental relations, international migration policies, domestic politics and social interactions. In the coming years, same-sex marriage will remain a controversial and salient part of the legal, political and cultural landscape locally, nationally and internationally.