(UNDATED) The California Supreme Court upheld a statewide ban on gay marriage on Tuesday (May 26) but preserved the estimated 18,000 gay marriages that were performed before voters approved the ban last November.

By a 6-to-1 majority, the court said that by approving Proposition 8 last fall, California voters had spoken clearly that marriage should be reserved for heterosexual couples.

A number of religious groups, including the Roman Catholic Church and the Church of Jesus Christ of Latter-day Saints, used their money and influence to push for Prop 8, which was backed by 52 percent of California voters.

Chief Justice Ronald George, author of the court’s majority decision, said same-sex couples maintain the right to form civil unions, which are “officially recognized, and protected family relationship(s)” with “all of the constitutionally based incidents of marriage.”

Still, those civil unions will differ — in name at least — from the estimated 18,000 still legitimate marriages performed between the court’s decision to legalize gay marriage last May and the passage of Proposition 8.

Gay rights leaders had argued that Prop 8 amounted to a “revision” to California’s constitution, and thus needed the approval of two-thirds of the state legislature before it went on the ballot last fall.

The California ruling comes on the heels of a string of victories for gay rights advocates, as Iowa, Vermont and Maine have legalized gay marriage in recent months. Similar initiatives are underway in New Hampshire, New York and New Jersey. Gay marriage is currently legal in five states; 29 states have constitutional prohibitions against gay marriage or same-sex unions; 12 more have statutory bans on gay marriage.

The California ruling is a bad omen for gay rights advocates in the 29 states that have passed constitutional amendments banning gay marriage, said Robert Tuttle, a professor at the George Washington University Law School. If other state courts take similar approaches, the only way to overturn the bans is through costly voter referendums that have not been effective to date.

“You’re going to quickly run out of states where you can do anything through the legislature,” as advocates in New England have successfully done in recent months, Tuttle said.

Gay rights groups acknowledged Tuesday’s setback but said momentum and poll numbers are increasingly on their side. For the first time, a Washington Post/ABC News poll in April found more Americans supporting gay marriage than opposing it (49 to 47 percent). Several gay rights groups pledged to bring a new referendum to overturn the California ban on gay marriage before voters as soon as 2010.

“I’m very saddened,” said the Rev. Troy Perry, founder of the predominantly gay Metropolitan Community Church, whose lawsuit helped lead to last year’s approval of same-sex marriage. “The battle is not over. As far as I’m concerned we have won the battle but we have … skirmishes that have to take place.”

Brad Luna, communications director for the Human Rights Campaign, said the gay rights group has formed strategic partnerships with faith leaders to mobilize support for marriage equality. Polls showed that 85 percent of evangelicals, 66 percent of other Protestants and 60 percent of Catholics approved the ban on gay marriage — numbers that HRC is determined to shift, Luna said.

“One thing we saw from the polling are the divisions between people of faith on this issue,” said Luna. “Putting forth faith leaders speaking up for loving, committed couples is an important voice to make sure is out there.”

Tony Perkins, head of the conservative Washington-based Family Research Council, praised part of the court decision, saying “even this widely-recognized liberal court understands that overturning Proposition 8 would represent a repudiation of the state constitution it is sworn to uphold.”

But Perkins criticized the court’s decision to keep intact the 18,000 same-sex marriages performed before Prop 8 passed last year.

“The court’s recognition of these `marriages’ clearly seeds the ground for a possible legal battle before the U.S. Supreme Court,” he said in a statement on Tuesday.

(David Finnigan contributed to this report from Los Angeles.)