Ruling in favor of a challenge to the law restricting marriage to one man and one woman, King County Superior Court Judge William L. Downing said there is no evidence that same-sex marriage threatens youngsters in nontraditional families.

"Although many may hold strong opinions on the subject, the fact is that there are no scientifically valid studies tending to establish a negative impact on the adjustment of children raised by an intact same-sex couple as compared with those raised by an intact opposite-sex couple," Downing wrote.

Barring same-sex marriages serves no "state interest" and violates the constitutional right of gay couples to due process, he ruled.

The state Supreme Court must review the case before same-sex marriage licenses can be issued, said Jennifer Pizer, lead counsel in the case for Lambda Legal Defense.

"Judge Downing saw the couples in the courtroom and he's recognized that they are full and equal citizens of Washington," Pizer said.

Arguing for the couples, attorney Bradley Bagshaw told Downing at a hearing last month that the act violates the state constitution by depriving same-sex couples of the same rights as other residents.

Six couples filed the lawsuit in March after King County refused to grant them marriage licenses, and two other couples later joined the suit.

A second lawsuit was filed in April by the American Civil Liberties Union on behalf of 11 same-sex couples.

Downing concluded that barring same-sex partners from civil marriage is "not rationally related to any legitimate or compelling state interest."

Critics of same-sex marriage criticized the ruling.

"I'm disappointed that we even have to be deliberating a well-precedented matter that people previously defined as a marriage between a man and a woman," said state Sen. Val Stevens, a Republican who intervened in the case. "What's to say we can't call a sister-brother union marriage? Where do you draw the line?"

King County Executive Ron Sims, a defendant in the lawsuit, said the ruling was a powerful affirmation of equal rights.

"I always believed that if the court of law addressed this that the court would conclude that the prohibition against gays and lesbians from being married would be found unconstitutional," Sims said. "I think marriage is an incredibly wonderful institution and that people who love each other should be allowed to be involved in it."

When first urged to issue marriage licenses to gay couples, Sims refused and said the licenses would have no legal meaning in a state that did not recognize them.

Washington is one of 38 states with laws defining marriage as a union between a man and a woman. Under a state high court ruling, Massachusetts has allowed gay marriage since May.