The first same-sex marriages in the U.S. took place in Massachusetts on May 17, 2004. This was six months after the state's Supreme Court ruled that a ban on such unions was unconstitutional. Since then, eleven other states and Washington, D.C. have also made gay marriage legal. (The Netherlands was the first country to legalize gay unions in 2001.)
For three months in 2008, the state of California also allowed same-sex marriages, until voters narrowly voted against them later that year. However, any marriages that were performed in those months are still valid. In August 2010 a Federal District Court in San Francisco overturned Proposition 8 but resumption of marriages is on hold pending appeal. (The case was argued in front of the U.S. Supreme Court in March 2013.)
The latest marriage figures for Massachusetts are through 2010 and they show that more than 13,000 same-sex couples had gotten hitched, comprising 7% of all marriages in the state during that period.