LANSING, Michigan (AP) – Michigan’s ban on same-sex marriage also blocks public universities and state and local governments from providing health insurance benefits to the partners of gay workers, the state Court of Appeals ruled Friday.

A three-judge panel cited the language of a 2004 voter-approved constitutional amendment making the union between a man and woman the only agreement recognized as a marriage “or similar union for any purpose.”

“The protection of the institution of marriage is a long-standing public policy and tradition in the law of Michigan,” wrote Judges Kurtis Wilder, Joel Hoekstra and Brian Zahra.

Twenty-one same-sex couples will appeal to the state Supreme Court. If they are not successful, they will sue in federal court, said attorneys for the American Civil Liberties Union of Michigan.

“It’s hugely disappointing,” said Gary Lindsay, an office assistant at Michigan State University whose partner of 17 years gets health coverage along with family medical and bereavement leave. “That’s one of the primary reasons for working at this university – the fact that they are open to people’s diversity and they strive to treat people fairly.”

Besides Lindsay, 15 of the plaintiffs work for employers who offer same-sex benefits – the city of Kalamazoo, various universities and a county health department covering the Lansing area. Another five plaintiffs are employed by the state, which in 2004 agreed to provide same-sex benefits but delayed them until courts ruled on their legality.

The unanimous appeals court ruling released Friday reverses a 2005 decision by an Ingham County judge who allowed public employers to provide the benefits. Circuit Judge Joyce Draganchuk said criteria established by employers to qualify for same-sex benefits were not akin to marriage because married people get many more rights than domestic partners.

Republican Attorney General Mike Cox, whose legal opinion barring same-sex benefits in future contracts prompted the ACLU lawsuit, said the constitutional amendment is unambiguous.

“The people set the direction. They were pretty clear,” he said in a telephone interview. “In Michigan, government cannot use anything that looks like a marriage as a basis for giving out benefits if it isn’t actually a marriage.”

The ruling also applies to unmarried heterosexual couples.

Gay rights advocates and Democratic Gov. Jennifer Granholm said the public never intended to ban same-sex benefits. But the appeals court said exploring voters’ intent was unnecessary because the law is clear.

“It is a cornerstone of a democratic form of government to assume that a free people act rationally in the exercise of power, are presumed to know what they want, and to have understood the proposition submitted to them in all of its implications,” the judges wrote.

Massachusetts is the only U.S. state to allow same-sex marriage, with Connecticut, Vermont and New Jersey offering civil unions. California and Hawaii have domestic partnerships that allow some of the same rights as marriage.