Michigan has gone farthest, prohibiting cities, universities and other public employers from offering benefits to same-sex partners. In all, 27 states have passed constitutional amendments defining marriage as the legally sanctioned union of a man and a woman.

A Michigan court ruled in February that public employers may not offer benefits to unmarried partners, gay or straight, because of a 2004 amendment defining marriage. Government employers there had offered benefits only to gay couples.

Kalamazoo and the Ann Arbor school district have notified employees that they will end domestic partners' benefits. An appeal is before the state Supreme Court.

Kentucky Attorney General Gregory Stumbo ruled this month that the University of Kentucky and the University of Louisville may not offer benefits to domestic partners, gay or straight.

A U.S. appeals court last year upheld Nebraska's amendment barring government employers from granting benefits, including health insurance, to same-sex couples. It didn't address benefits for unmarried heterosexual couples.

Ohio state Rep. Tom Brinkman, a Republican, has filed a lawsuit to bar Miami University of Ohio from offering benefits to same-sex partners of employees.

"We're in kind of a giant race, a historic race, with all these court cases," says Matt Daniels, president of Alliance for Marriage, which lobbies for a marriage amendment to the U.S. Constitution. "When the dust settles, we'll have a national standard for marriage. What is going on in the states is a dress rehearsal."

Gay-rights activists say they are fighting for families, too.

"Anti-gay organizations have tried to attack currently existing protections for gays and lesbians and unmarried couples for a long time," says Camilla Taylor, an attorney for the gay-rights organization Lambda Legal. "They don't want to limit marriage between a man and a woman — they want to attack the protections that exist and make life difficult for non-traditional families."

Most of the 27 state amendments were passed after a 2004 Massachusetts law allowed gay marriage. An additional 17 states passed marriage laws but did not amend their constitutions.