By Bernie Woodall

DETROIT (Reuters) - Michigan's ban on same-sex marriage violates the U.S. Constitution and must be overturned, a federal judge ruled on Friday in the latest in a series of court decisions to allow gay couples to wed.

Michigan Attorney General Bill Schuette said he was seeking an emergency stay and appeal of the ruling with the U.S. Court of Appeals for the Sixth Circuit, which struck down a Michigan constitutional amendment adopted by voters in 2004.

The ban "does not advance any conceivable legitimate state interest" and discriminates against same-sex couples in violation of their right to equal protection, Judge Bernard Friedman found in a 31-page ruling.

The challenge to Michigan's law was brought by April DeBoer and Jayne Rowse, a lesbian couple who live in Hazel Park, a Detroit suburb. They had sought to jointly adopt each other's children, were denied under Michigan law, and then challenged both the ban on same-sex marriage and state adoption law.

DeBoer told a news conference that she and Rowse would not marry right away because of the possibility that the decision could be overturned.

"We are going to get married when we know we can stay married," DeBoer said.

Friedman's ruling followed a nine-day trial and tracked along the lines of recent rulings by federal judges who found bans on gay marriage unconstitutional in Texas, Utah and other states. Those rulings have been put on hold pending appeals.

While Schuette was seeking an emergency order to stay Friedman's ruling, county clerks around Michigan were preparing to issue marriage licenses on Monday morning.

"I knew we'd see this day, but it was a long time coming," said Susan Horowitz, publisher of Between the Lines, a gay advocacy publication in Michigan.

Horowitz said she planned to marry her long-time partner on Monday in Oakland County, which includes Detroit suburbs. They were married in Ontario in 2005, but want to marry in her home state, she said.

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Barely a decade ago no U.S. states permitted gays to marry. Support has surged since Massachusetts became the first state to make same-sex marriage legal and it has extended to 17 states plus the District of Columbia.

Momentum has grown since the U.S. Supreme Court ruled in June that legally married same-sex couples are eligible for federal benefits in a decision that struck down a key part of the federal Defense of Marriage Act.

Gay rights advocates also have filed lawsuits challenging bans in Arizona, Indiana, Wisconsin, Wyoming and other states.

(Reporting by Bernie Woodall in Detroit and David Bailey in Minneapolis; Editing by Cynthia Johnston, Andrew Hay, Sharon Bernstein and Lisa Shumaker)