A judge in Florida overturned the state's ban on same-sex marriage on Thursday.
The ban had been approved by 62 percent of voters in 2008.
"The court is aware that the majority of voters oppose same-sex marriage, but it is our country's proud history to protect the rights of the individual, the rights of the unpopular and rights of the powerless, even at the cost of offending the majority," Monroe County Circuit Judge Luis Garcia wrote in his opinion.
Florida Attorney General Pam Bondi filed an appeal almost immediately.
Garcia's decision applies only to Monroe County, which includes Key West. Gay couples there will be able to marry next Tuesday, Garcia having allowed county clerks a few days to prepare.
A similar case is pending in Miami-Dade County.
"We can now add Florida's voice to the urgent need for the Supreme Court to take a freedom to marry case and bring the entire country to national resolution, ending marriage discrimination across America," Evan Wolfson, a prominent same-sex-marriage advocate, said in a statement.
Attorneys for the state had argued that the will of the voters should be respected.
"With one stoke of a pen, a mere trial judge has attempted to overthrow an act of direct democracy by five million Floridians who defined marriage as the union of one man and one woman," John Stemberger, president of the Florida Family Policy Council, which was one of the primary backers of the 2008 measure, said in a statement.
But Garcia concluded that "a citizen's right to marry is a fundamental right that belongs to the individual."
His decision was the latest in a series of more than 20 victories for same-sex-marriage supporters since the Supreme Court ruled on the question last year. Earlier this month, a federal judge in Kentucky struck down that state's ban on gay marriage.