At least 10 countries - including Argentina, Spain, Norway, Sweden, Belgium and South Africa - have legalized same-sex marriage. At least 20 other countries recognize or perform civil unions - a status that typically confers most or all of the legal rights and privileges of marriage without deeming the union to be a true marriage.
In the U.S., five states - Connecticut Iowa, Massachusetts, New Hampshire and Vermont - and the District of Columbia have legalized gay marriage and several others either allow civil unions or legally recognize gay marriages performed elsewhere.
Proponents of same-sex marriage say that gay couples deserve equal treatment under the law (a right protected in the U.S. Constitution) including the financial benefits of marriage in the eyes of the law and rights like hospital visitation and next-of-kin status. They also assert that the denial of the right to marry stigmatizes homosexuality, potentially contributing to anti-gay bigotry and crime and metal health issues among gay people.
Opponents say that the word "marriage" is intrinsically defined as comprising a relationship involving one man and one woman, that it is a religious institution that can be policed by each religion, and - in the case of extreme opponents - that it is immoral or sinful and results in non-functional, corrupting families.
That division of opinion has been evident in the passage and subsequent legal wrangling over Proposition 8, a California ballot initiative restricting marriage to heterosexual couples. The campaigning on both sides of Prop 8 was some of the most expensive and contentious in state history.
In August, 2010 a federal judge overturned the law on Constitutional grounds, but left enforcement of the ruling on hold pending the appeals process.