The first same-sex couples walked down the aisle in the Netherlands in 2001, with others following suit in Canada, South Africa, Belgium and Spain. Argentina was the first Latin American nation to legalize such marriages, in 2010.
Legislators in the United Kingdom are also weighing proposals to legalize same-sex marriage.
However, many countries remain split over the issue.
In the United States, the question went before the Supreme Court last month, and justices are now deliberating over the matter.
Nine states and the District of Columbia issue marriage licenses to same-sex couples, including three states -- Maryland, Washington, and Maine -- where voters approved it in ballot initiatives last year. Many states have specific laws blocking same-sex couples from legally marrying.
Lawmakers in Australia voted against a bill to legalize same-sex marriage last September. A poll for the advocacy group Australian Marriage Equality indicated that 64% of those surveyed "support marriage equality."
A law legalizing civil unions was introduced in 1999 in France under a previous Socialist government.
Known in France as the PACS, or pacte civil de solidarite, the civil union agreement can be entered into by same-sex or straight couples and confers many but not all of the rights of marriage.