Canada became the fourth nation to legalize same-sex marriage on July 20, 2005 (following the Netherlands, Belgium and Spain). By the time the legislation, known as the Civil Marriage Act, was approved, all but two of the country's provinces had already passed legislation in favor of same-sex unions in their jurisdictions (Ontario was the first province to do so on June 10, 2003). This came one year after Massachusetts became the first US state where same-sex marriages were legal.
In the ten years since then eighteen more nations have legalized same-sex marriage - including the United States. Back in 2012 President Obama announced his support of same-sex marriage; one year later the Supreme Court struck down the Defense of Marriage Act (DOMA); and in June 2015 the Court struck down all remaining state bans, making it legal nationwide. The combined population of those nations where same-sex marriage is legal is more than 900 million, about 13% of the world's population.