DETROIT – A federal judge Friday blocked Michigan’s ban on domestic partner benefits for employees who work for public schools or local governments, saying state lawmakers simply wanted to punish gays and lesbians.

U.S. District Judge David Lawson said plaintiffs who have lost benefits or were forced to buy expensive private health insurance have made a “plausible claim” that the law violates the Equal Protection Clause of the U.S. Constitution. The decision came nearly a year after he heard arguments in the lawsuit, filed by the American Civil Liberties Union.

“It is hard to argue with a straight face that the primary purpose — indeed, perhaps the sole purpose — of the statute is other than to deny health benefits to the same-sex partners of public employees. But that can never be a legitimate governmental purpose,” Lawson said as he ordered an injunction.

The law, passed in 2011 by the Republican-controlled Legislature and signed by Gov. Rick Snyder, ended insurance for people whose domestic partners work for certain public employers.

It’s somewhat narrow, exempting colleges and universities, as well as most state government workers whose benefits are set by the Michigan Civil Service Commission.

A handful of school districts had offered benefits before the law took effect in 2012, along with Ingham and Washtenaw counties and the cities of Ann Arbor, East Lansing and Kalamazoo, according to the ACLU.

Supporters of the law say it saves tax dollars and follows the spirit of a 2004 constitutional amendment, approved by 58 percent of voters, that defines marriage only as a union between a man and a woman.

Lawson, however, said that argument stinks. The state’s claims “come close to striking (the court) with the force of a 5-week-old, unrefrigerated dead fish,” he said.

In his 51-page opinion, Lawson cited this week’s decision by the U.S. Supreme Court that struck down a portion of a federal law that barred certain benefits to married same-sex couples.

“This is great news. It’s a very mean-spirited law,” said Lansing piano teacher Gerado Ascheri, 55, who has been paying $460 a month after losing better, less expensive health coverage through his partner’s employer, Ingham County.

Barbara Ramber, 54, said she and partner JoLinda Jach, 49, have paid more than $4,000 in out-of-pocket expenses since Ramber lost insurance through the city of Kalamazoo. She has glaucoma and other eye ailments, but her replacement insurance isn’t generous.