Six gay and lesbian couples on Thursday filed a federal lawsuit challenging South Dakota's ban on gay marriage, leaving North Dakota as the only state without an active lawsuit challenging the state's marriage ban.

Lawmakers in 1996 approved a law prohibiting same sex couples from marrying. A decade later, voters approved a constitutional amendment reinforcing the law.

Five of the plaintiff couples want their out-of-state marriages recognized by South Dakota. The sixth couple was denied a marriage license on Thursday. Two of the couples are raising children.

Attorney General Marty Jackley, a Republican, has already said that he will defend the ban in court.

“It is the statutory responsibility of the attorney general to defend both our state constitution and statutory laws, which I intend to do if a lawsuit is filed,” Jackley told the AP earlier this month.

Gay couples can wed in 19 states, in addition to the District of Columbia, with Oregon and Pennsylvania being the latest.