NEW YORK — A federal judge in Detroit struck down Michigan’s ban on same-sex marriage Friday, the latest in a string of court decisions across the country to rule that denying marriage to gay and lesbian couples is a violation of the Constitution.
“The guarantee of equal protection must prevail,” wrote District Court Judge Bernard A. Friedman.
But unlike cases in other states, Friedman did not suspend his decision while the Michigan attorney general pursues an appeal. The Associated Press reported that Washtenaw County, which includes Ann Arbor, said it would start issuing licenses Saturday for four hours unless a higher court intervenes.
Ingham County, where the state capital is located, planned to follow suit on Monday.
Attorney General Bill Schuette said he would immediately ask the US Court of Appeals for the Sixth Circuit, in Cincinnati, to freeze Friedman’s decision and prevent same-sex couples from marrying while he appeals the case, the AP reported.
The two-week trial, which ended March 7, drew special attention because it was the first in several years to include testimony from social-science researchers on the potential impact of same-sex marriage on families and children.
The state, arguing that it would be risky to change the definition of marriage, cited studies concluding that children raised by same-sex couples had worse outcomes in life. Those challenging the ban argued that the studies had been widely discredited, and Friedman agreed, calling them deeply flawed.
Lawyers for the plaintiffs described the scholars who appeared for the state as religiously motivated and part of a “desperate fringe,” and subjected them to withering cross-examination.
Scholars called to rebut the testimony described a virtual consensus in the field that other things, such as income and stability, being equal, children fared just as well with same-sex parents.
The unusually extensive trial record could strengthen Friedman’s ruling in the appeals process and could prove useful elsewhere as dozens of similar challenges to marriage restrictions enter state and federal courts. Friedman was appointed to the federal seat in the eastern district of Michigan in 1988 by President Ronald Reagan.
The Michigan attorney general also argued that it was wrong for federal courts to overturn a policy adopted by public referendum in 2004, and that Michigan voters should decide if change was needed. But Friedman ruled that state authority “cannot trump federal constitutional limitations.”
The case was brought by April DeBoer, 42, and Jayne Rowse, 42, nurses who live together in a Detroit suburb and who each adopted children born with special needs and are raising them as one family.
They originally sued in 2012 to overturn a state law that prevents them, as unmarried parties, from adopting their three children as a couple. They wanted to do so to protect their parental interests in the event one of them dies.
They later expanded their suit to challenge the underlying obstacle — Michigan’s ban on same-sex marriage or even civil unions.
Since June, when the US Supreme Court struck down part of the federal Defense of Marriage Act, federal judges in Utah, Oklahoma, Virginia, Texas, and now Michigan have declared that state amendments and laws limiting marriage to a man and a woman are unconstitutional.
The rulings are headed for circuit courts of appeal in coming months, putting pressure on the Supreme Court to revisit the issue.
Seventeen states and the District of Columbia issue licenses for same-sex marriage. Massachusetts became the first state in the nation to legalize same-sex marriage in 2004, when the state Supreme Judicial Court ruled that limiting marriage to heterosexual couples violated the state Constitution.
In his decision, Friedman noted that supporters of same-sex marriage believe the Michigan ban was at least partly the result of animosity toward gays and lesbians.
‘‘Many Michigan residents have religious convictions whose principles govern the conduct of their daily lives and inform their own viewpoints about marriage,’’ Friedman said. ‘‘Nonetheless, these views cannot strip other citizens of the guarantees of equal protection under the law.’’
Conservative scholars testifying for the state questioned the impact of same-sex parenting on children, but the judge was unmoved by those arguments. ‘‘State defendants lost sight of what this case is truly about: people,’’ Friedman said. ‘‘No court record of this proceeding could ever fully convey the personal sacrifice of these two plaintiffs who seek to ensure that the state may no longer impair the rights of their children and the thousands of others now being raised by same-sex couples.’’
The decision was filed shortly after 5 p.m. in Detroit, when most county clerk offices were closed.