Jennie Rosenkranz, 72, left, and Nancy Robrahn, 68, are the lead plaintiffs in the South Dakota suit.  The two have taken the last name Rosenbrahn. (Benjamin Brayfield/Rapid City Journal via AP)

North Dakota’s same-sex marriage ban is the last remaining state ban not taken to court — for now.

Six couples sued South Dakota over its same-sex marriage ban  Thursday afternoon, their lawyer told The Washington Post. The ban in neighboring Montana was sued Wednesday.

The prospect of every state same-sex marriage ban being brought to court is noteworthy because none of the 13 challenges that have made it to federal court since last summer has survived unscathed. Federal judges have struck down ten state bans and forced another three states to recognize at least some out-of-state marriages.

The suit was filed by Joshua Newville, a lawyer with the Minneapolis-based firm Madia Law. Newville says he has been approached by couples in North Dakota, though he has yet to decide whether he will bring a challenge against that state’s law. If he does, it will be the last to fall.

North Dakota is home to the fewest self-identified lesbian, gay, bisexual or transgendered individuals, according to a 2012 Gallup poll. Just 1.7 percent of those interviewed identified as LGBT. Montana, Mississippi and Tennessee were ranked second, each with a 2.6 percent response rate.

Both Dakotas lack laws that specifically protect against discrimination based on sexual orientation, meaning couples filing suit may be subject to some form of retaliation. The lead plaintiffs in the South Dakota case, the newly wed Rosenbrahns, felt comfortable with that suit because own their own home and business, Nancy Rosenbrahn said.

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South Dakota same-sex marriage lawsuit