Voters who identified themselves as Catholics and Protestants combined to help pass Proposal 2, defining marriage as a union between one man and one woman in the state constitution.
But opponents, concerned about the proposal's potential impact on domestic partner benefits, said they will spend the next few days determining their possible legal options.
With 94 percent of precincts reporting early Wednesday, 2,549,603 voted yes, or 59 percent, and 1,798,602, or 41 percent, voted no.
Gogebic County overwhelmingly supported the measure, with 5,254 voting yes and 2,662 voting no. In Ontonagon County, there were 2,680 yes votes to 1,277 no votes.
"We were supported across the board," said Marlene Elwell, campaign chairwoman for a pro-Proposal 2 group called Citizens for the Protection of Marriage. "We didn't find any area in the state where we weren't supported."
Jeffrey Montgomery, executive director of the Triangle Foundation, called Tuesday's vote an attempt to put discrimination in the state constitution.
"It will be seen as a dark day in our history," Montgomery said.
A poll of 2,443 voters conducted for The Associated Press and television networks by Edison Media Research and Mitofsky International showed broad support for Proposal 2 among blacks and whites. It was supported by nearly two-thirds of men and a smaller majority of women.
Nearly two-thirds of voters who identified themselves as Catholic, Protestant or other Christian supported the measure. But those who identified themselves as members of a different religion or had no affiliation tended to be opposed. The poll had a margin of error of 3 percentage points.
Voters also split along party lines. Those who identified themselves as Republican strongly favored the proposal, while a smaller majority of Democrats were opposed. Independents favored the proposal.
Voters from all income levels favored the proposal, as did all age groups except voters between 18 and 29, who were evenly divided.
Supporters say the constitutional amendment will protect against judges or politicians allowing same-sex marriage in Michigan.
"That's the way God wanted it, isn't it?" said Latasha Lopez, 25, of Detroit.
But opponents said the proposal is worded so broadly it could eventually ban domestic partner benefits for both gay and straight couples.
"To make alterations to a constitution that was well-thought out in the first place is a serious proposition," said George Crockett III, 65, a retired Wayne County judge.
Supporters said they were pushing the proposal as a way to strengthen families. They became alarmed when the state of Massachusetts legalized gay marriages early this year after a series of court rulings.
Michigan law already defines marriage as a union between one man and one woman, but supporters said putting language in the constitution would make it tougher to change.
Opponents said it was an unnecessary amendment that could encourage discrimination against gays and unmarried straight couples in Michigan. Opponents gained support from several groups including the Michigan AFL-CIO, the Michigan Education Association and the Michigan Civil Rights Commission.
The pro-Proposal 2 campaign got more than $1.3 million in financial support. More than half of that came from the Catholic Church, which has 2.25 million members in Michigan -- nearly a quarter of the state's population.
The Catholic Church actively encouraged its members to vote for the proposal, but support among parishioners was not uniform.
Among the Catholics breaking ranks with the church establishment was Gov. Jennifer Granholm. She did not actively campaign against Proposal 2, but if asked, her office would send a letter to a constituent outlining her opposition to the measure. Granholm wrote that the measure was unnecessary and would have unintended negative consequences.
The main committee opposing the measure, the Coalition for a Fair Michigan, raised more than $700,000 as of late last week. Most of the money arrived after mid-October.