Washington D.C.

Washington, D.C., the capital of the United States, became the sixth location, besides five other states, to legalize same-sex marriage. It became legal on March 3, 2010, after a bill signed by Adrian Fenty, Mayor of Washington, D.C., and was passed by the council of the District of Columbia. The bill was originally passed in December but had to undergo a review of 30 days.
http://www.webcitation.org/5nx4LhJxv

From 1992 until now, Washington, D.C. has permitted that resident same-sex couples enter into domestic partnerships. Since 2009 it has also recognized same-sex marriages from other states in the U.S.
http://www.webcitation.org/5nx4LhJxv
http://www.glaa.org/archive/2001/dpfactsheet103001.shtml

With the allowance of same-sex marriage, Washington, D.C. became the first location below the Mason-Dixon Line to allow same-sex marriage.
http://www.ebar.com/news/article.php?sec=news&article=4589