Massachusetts became the first state in the U.S. to allow same-sex marriage after it was ruled by the Supreme Judicial Court of Massachusetts that under the Massachusetts Constitution it was unconstitutional to allow only heterosexual couples to marry. Same-sex marriages were then officially allowed in May of 2004.
http://www.boston.com/news/local/massachusetts/articles/2003/11/18/sjc_gay_marriage_legal_in_mass/
Connecticut became the third state in the U.S. to allow same-sex marriage (California was the second state to do so, but the allowance was subsequently repealed, making Connecticut the second state to allow same-sex marriage and not have it repealed) when it became legal on October 28, 2008.
http://www.nytimes.com/2008/10/11/nyregion/11marriage.html?pagewanted=all
Iowa legalized same-sex marriage on April 3, 2009.
http://www.desmoinesregister.com/article/20090403/NEWS/90403010/Unanimous-ruling-Iowa-marriage-no-longer-limited-one-man-one-woman
Lambda Legal filed a lawsuit against the State of Iowa for six same-sex couples who argued that not being allowed the same rights of heterosexual couples violated their rights in the liberty and equal protection clauses of the Iowa State Constitution. After the Polk County District Court ruled in favor of the couples in 2007, the case was brought to the Iowa Supreme Court and the ruling of the county court was unanimously upheld, and same-sex marriage became legal.
http://www.nytimes.com/2007/08/31/us/31iowa.html
Vermont began allowing same-sex marriage on September 1, 2009. It became the fourth state in the U.S. to allow same-sex marriage, and the first state to do so without court intervention.
http://www.guardian.co.uk/world/deadlineusa/2009/sep/01/gay-marriage-vermont
http://www.vpr.net/news_detail/84626/
Before allowing same-sex marriage Vermont became the first state in the U.S., in 2000, to introduce civil unions. The civil unions allowed for same-sex couples to have the same rights as married couples without officially being "married."
http://www.religioustolerance.org/hom_mar8b.htm
New Hampshire began allowing same-sex marriage on New Year's Day 2010.
http://www.gencourt.state.nh.us/legislation/2009/HB0436.html
New Hampshire first legalized civil unions in 2008. With the allowance of same-sex marriage two years later, the former civil unions automatically became marriages unless they'd been previously dissolved or annulled.
http://www.concordmonitor.com/article/nh-house-passes-civil-unions?CSAuthResp=1319734662%3Av3i7j1oqgv6vk0palj4p3rodk5%3ACSUserId|CSGroupId%3Aapproved%3AC115F488652EFFD40E17F740917B7DE9&CSUserId=94&CSGroupId=1
http://www.concordmonitor.com/article/civil-unions-six-months-later?CSAuthResp=1319734847%3Av3i7j1oqgv6vk0palj4p3rodk5%3ACSUserId|CSGroupId%3Aapproved%3A5D6B37F3136BA6DD46FF69A7B8CD0022&CSUserId=94&CSGroupId=1
And finally. . .Maine.
In May 2009 Gov. John Baldacci signed into law a bill that would make same-sex marriage in Maine legal. The bill however, would have to wait as opponents petitioned for a referendum and were ultimately successful. The referendum to disallow same-sex marriage was brought up in the 2009 general elections and was consequently approved, repealing the bill that had previously been passed.
http://www.maine.gov/sos/cec/elec/2009/referendumbycounty.html
http://www.politico.com/news/stories/1109/29119.html
