Six couples filed a federal lawsuit against South Dakota's gay marriage ban today, leaving North Dakota as the only state in the country with an unchallenged same-sex marriage ban.

Same-sex marriage bans have been falling fast since the U.S. Supreme Court struck down the Defense of Marriage Act last June. As of today, gay marriage is legal in 18 states and Washington, D.C. On June 1, Illinois will be the 19th state (same-sex marriage is already legal in certain counties there). Just this week, Oregon and Pennsylvania became the newest states to legalize same-sex marriage.

In seven other states, the ban was struck down by a court but the ruling was stayed pending appeal. In the rest (except North Dakota), a legal challenge either to the state's ban or its failure to recognize the rights of gay couples married in other states is pending.

Here it is in map form:

Same-Sex Marriage Legality as of May 22, 2104 Map Template via Google

This is, of course, subject to change, possibly very soon. Josh Newville, the lawyer representing the South Dakota couples, told the AP that he's been approached by several gay couples from North Dakota and is "seriously considering" taking their case on. 

North Dakota has the fewest lesbian, gay, bisexual or transgender residents proportional to its total population, according to a 2012 Gallup poll, with just 1.7 percent of those polled identifying as such. This was well below the second-to-last percentage: 2.6, in Mississippi, Montana and Tennessee. Hawaii, with 5.1 percent, had the most.