North Dakota is the last state in the country to be sued by same-sex couples seeking the right marry in the state.
Seven couples filed a federal lawsuit Friday challenging North Dakota's constitutional amendment defining marriage.
The lawsuit, filed in U.S. District Court in Fargo, challenges both North Dakota's refusal to recognize marriages of same-sex couples who legally wed in other states.
That means cases are currently pending in all 31 states that still do not recognize gay marriage. Judges have overturned several of those amendments since the U.S. Supreme Court struck down part of the federal Defense of Marriage Act last year.
North Dakota attorney general's office said it had not yet seen the suit and was not commenting.
The 2004 voter-approved constitutional amendment defining marriage as between a man and a woman was passed by 73 percent of voters.