North Dakota is the last and only state in the country with an unchallenged ban on marriage equality. Every other state has had its ban struck down in federal court (with a pending appeal), is in the middle of having its ban challenged in court or has marriage equality.
And North Dakota may see a legal challenge sooner than later. Josh Newville, the lawyer representing the South Dakota couples who filed their court case on Thursday, told the Associated Press that he has been approached by couples in North Dakota about challenging the state ban and is "seriously considering" taking on their case.
Mother Jones has a pretty neat interactive map you can check out here.