PHOENIX — Attorneys for the state of Arizona say in court documents that the state’s ban on same-sex marriage is justified because same-sex couples cannot reproduce without the help of a third person.

In documents filed Wednesday in a federal challenge to the constitutional ban approved by voters in 2008, the state says its public purpose in regulating private relationships is to ensure that children are, whenever possible, raised by a biological mother and biological father.

“Only man-woman couples are capable of furthering the state’s interest in linking children to both of their biological parents,” argue attorneys from the Scottsdale-based Alliance Defending Freedom (ADF).

They say the vast majority of such couples produce their own biological children, and that same-sex marriages “do not advance that compelling state interest.”

While the lawsuit is against the state, the case is being defended by the ADF, a self-described “legal ministry” formed by Christian leaders to advocate for religious liberty and marriage.

Attorney General Tom Horne agreed to let ADF take the lead, naming the lawyers as special assistant attorneys general, according to the Arizona Capitol Times.

Plaintiffs say the state’s arguments on procreation is “holds no water,” at least in part because Arizona recognizes marriage between couples who are infertile or do not wish to procreate.

The suit, filed in January in U.S. District Court in Phoenix on behalf of four same-sex couples, claims a voter-approved ban on gay marriage is unconstitutional because it violates their rights of equal protection and due process. It seeks to allow same-sex couples to be married and recognize same-sex marriages conducted in other states.

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Two of the plaintiff couples were married in California. All are long-time Arizona residents and two of the couples have adopted children, according to their attorney Shawn Aiken.

The case is one of two federal challenges to Arizona’s same-sex marriage ban. The second suit was filed in March by Lambda Legal on behalf of seven same-sex couples and the surviving spouses of two additional same-sex couples.

Both cases are assigned to U.S. District Judge John Sedwick, a federal judge for Alaska who is among various federal judges who also hear cases in Arizona because of the Arizona court’s heavy workload. In April, Sedwick declined to consolidate the two cases.

The case is Connolly v. Roche.