"I think something that the best-intentioned backers of Prop 8 don't understand is that it basically hurts," said Jordan Krueger, 29, of Los Angeles, who is engaged to his partner Hank, 40.
"It has been a difficult struggle, going back and forth, always in limbo," he said. "But whether or not I can be married to my partner really has no impact on others' marriages and that's what the court said today."
The California case, which involved a 13-day trial that concluded in June, was the first in federal court to challenge a state prohibition on same-sex marriage.
Brown of the National Organization for Marriage, which has helped finance the legal case supporting Prop 8, said he expected the decision, and believes it will revive a nationwide debate.
"Judge Walker just fired the first salvo in a major culture war, and the latest example of a court crossing the line... People won't stand for marriage being redefined," he said.
Attorney Andy Pugno, of the conservative advocacy group Protect Marriage, and former Justice Department lawyer Charles Cooper have been defending Proposition 8, after neither California Attorney General Jerry Brown nor Gov. Arnold Schwarzenegger moved to defend the measure on behalf of the state.
Representing the plaintiffs is a pair of unusual bedfellows: former Republican Solicitor General Ted Olson and Democrat David Boies. The two prominent attorneys opposed each other before the Supreme Court in the case of Bush v. Gore, which decided the persidential election in 2000.
A California Field poll of registered voters last month found 51 percent support legalizing gay marriage with 42 percent opposed.
Nationwide public views are more narrowly divided, according to the most recent ABC News/Washington Post poll. Forty-seven percent of Americans polled favor gay marriage while 50 percent are opposed.
Five states -- Massachusetts, Iowa, Connecticut, Vermont and New Hampshire -- and the District of Columbia currently perform same-sex marriages. Four states recognize marriages performed elsewhere, and nine states grant civil unions or partnerships.
ABC News' Alex Stone contributed to this report.