(CNN) -- An Iowa district court ruled Thursday that same-sex couples can marry based on the state constitution's guarantee of equal treatment, court documents show.

An Iowa district judge ruled in favor of same-sex marriage.

The ruling was in response to a December 2005 lawsuit brought by six same-sex couples seeking to wed. They were denied marriage licenses and claimed such treatment violates equal-protection and due-process clauses in the Iowa constitution.

The court also struck down a state law declaring valid marriages are only between a man and woman.

The Iowa District Court for Polk County advances the case to the Iowa Supreme Court which will make a final decision on same-sex marriage, according to Lambda Legal, a gay and lesbian legal organization representing the couples.

The 63-page ruling, written by Judge Robert Hanson states: "Couples, such as plaintiffs, who are otherwise qualified to marry one another may not be denied licenses to marry or certificates of marriage or in any other way prevented from entering into a civil marriage pursuant to Iowa Code Chapter 595 by reason of the fact that both persons compromising such a couple are of the same sex."

The law describing marriage as between a man and a woman, "constitutes the most intrusive means by the state to regulate marriage. This statute is an absolute prohibition on the ability of gay and lesbian individuals to marry a person of their choosing," Hanson wrote.

Lambda says the six couples are all in long-term relationship - one couple has been together for six years, another couple has been together for 17 years.

"Three of the couples are raising children, others are planning families, and all want the responsibilities of marriage and the protections only marriage can provide," according to the organization.

"We respectfully disagree with the court's decision, and we're going to ask for a stay," said Polk County Attorney John Sarcone.

He said his office will examine whether it's best to file a motion to reconsider. But barring a change in the court's opinion, Sarcone will appeal the ruling.

Co-counsel for the plaintiffs along with Lambda Legal, Dennis Johnson called the ruling "a significant step forward in recognizing the constitutional rights of all Iowans, and it's an amazing day for same-sex couples and their families all across Iowa." E-mail to a friend

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