The Michigan Supreme Court ruled Wednesday that a 2004 ban against gay marriage also blocks governments and state universities from offering health insurance to the partners of gay workers.
The 5-2 decision affirms a state Court of Appeals ruling.
Up to 20 public universities, community colleges, school districts and local governments in Michigan have benefits policies covering at least 375 gay couples. Some of the plans began as far back as the early 1990s.
After the appeals court ruled in February 2007, universities and governments rewrote their policies to try to comply with the gay marriage ban -- so the effect of Wednesday's decision was unclear.
The new policies no longer specifically acknowledge domestic partnerships but make sure "other qualified adults," including gay partners, are eligible for medical and dental care. The adults have to live together for a certain amount of time, be unmarried, share finances and be unrelated.
The voter-approved law, which passed 59 percent to 41 percent, says the union between a man and woman is the only agreement recognized as a marriage "or similar union for any purpose."
Justice Stephen Markman, writing for the majority, said that while marriages and domestic partnerships aren't identical, they are similar. He was joined by Chief Justice Clifford Taylor and Justices Maura Corrigan, Elizabeth Weaver and Robert Young Jr.
Dissenting Justices Michael Cavanagh and Marilyn Kelly said the constitutional amendment prohibits nothing more than same-sex marriages or similar unions. They argued that circumstances surrounding the election suggest Michigan voters didn't intent to take away people's benefits.
Republican Attorney General Mike Cox in 2005 interpreted the measure to make unconstitutional existing domestic partner policies at the city of Kalamazoo and elsewhere.
Twenty-one gay couples sued, saying the amendment was about marriage and preserving the status quo -- not taking away benefits from gays. Democratic Gov. Jennifer Granholm sided with the couples.
The couples represented by the American Civil Liberties Union of Michigan argued the ballot committee that sponsored Proposal 2 "consistently and repeatedly" assured voters that the initiative was only about protecting marriage.
But Markman noted that both supporters and opponents of the amendment said ahead of time that benefits would be prohibited by the amendment.
"The role of this Court is not to determine who said what about the amendment before it was ratified, or to speculate about how these statements may have influenced voters," he wrote. "Instead, our responsibility is, as it has always been in matters of constitutional interpretation, to determine the meaning of the amendment's actual language."
Kelly countered that gay partners allowed to get health insurance aren't granted other rights, responsibilities or benefits of marriage -- equal rights to property, for instance.
"It is an odd notion to find that a union that shares only one of the hundreds of benefits that a marriage provides is a union similar to marriage," she wrote.