To date, 12 states have already legalized same-sex marriage: Massachusetts (first state to do so in 2003), Connecticut, Delaware, Maryland, Minnesota, New Hampshire, New York, Rhode Island, Vermont, Washington state, Maine, and Iowa. The District of Columbia also legalized gay marriage. Additionally, 8 states have certain legal provisions and civil unions for gay couples: California, Colorado, Hawaii, Illinois, Nevada, New Jersey, Oregon, and Wisconsin.
Several states are still against same-sex marriage, and 29 of them amended their constitutions to ban it.
For the interactive map, please visit: Pew Research Center: Same-Sex Marriage State-by-State