* Only marriage between a man and a woman valid-court
* "Shame on you," shout gay advocates outside court
* Gay advocates pledge to try again in California (Adds quote from opinion, reaction)
By Alexandria Sage
SAN FRANCISCO, May 26 (Reuters) - California’s supreme court backed a ban on gay marriage on Tuesday, upholding a voter-approved proposition defining marriage as between a man and a woman, but said the marriages last year of 18,000 same sex couples were still legal.
The court, which last year unexpectedly opened the door to same-sex unions in the most populous U.S. state, bowed to the majority of California voters who passed the ban known as Proposition 8 last November.
Gay marriage backers vowed to continue the fight at the ballot box in 2010, and more than a hundred supporters blocked San Francisco streets in a show of peaceful civil disobedience.
The court said the roughly 18,000 marriages that took place in the state before the November ban remained valid since the ban was not retroactive. That left the state of 37 million people with a tiny group of married same-sex couples that cannot grow.
"Only marriage between a man and a woman is valid or recognized in California," Chief Justice Ronald George wrote in the court’s opinion, arguing that the decision did not end broad protections for same-sex couples to form families.
Tuesday’s ruling was unlikely to be the last move in what is seen as a pivotal state in U.S. culture wars.
Social conservatives applauded but in Los Angeles gay advocates promised to try to change the state constitution again -- to affirm gay marriage -- in a battle seen as soon as November 2010.
"There is a smear on our constitution and the only way to get around it is through the ballot box," Kate Kendell, executive director of the National Center for Lesbian Rights, told supporters.
‘SHAME ON YOU’
The focus of the court’s opinion was on the constitutionality of the vote last November. It rejected arguments that a majority could not vote away rights of a minority. The court also said -- countering arguments against the ban -- that the change in the state constitution was not substantial enough to require a tougher process for passage.
The passage of Prop 8 by a 52 percent majority last year, in the same election that put Democrat Barack Obama in the White House, bucked California’s reputation as a liberal trendsetter. It spurred nationwide protests by gay advocates and drew praise from social conservatives.
Gay rights advocates on the courthouse steps in San Francisco began shouting "Shame on you" as soon as the decision was made public, and blocked streets.
Gov. Arnold Schwarzenegger asked for peaceful and lawful protests in a statement. He predicted same-sex marriage would eventually prevail in California.
Tony Perkins, president of the conservative Family Research Council, predicted continued success for his group, which has prevailed against gay marriage in every state where the question has been put to a popular vote.
"This is not an issue that is going to go away," he said.
The California court on Tuesday did not back away from its sweeping decision last year, which held that same-sex couples had fundamental constitutional rights and deserved special legal protections as a minority class. Proposition 8 was put to voters as a result of that court decision.
The proposition’s single line, reading "Only marriage between a man and a woman is valid or recognized in California," was too narrow to invalidate fundamental rights, the court held.
"Proposition 8 reasonably must be interpreted in a limited fashion as eliminating only the right of same-sex couples to equal access to the designation of marriage, and as not otherwise affecting the constitutional right of those couples to establish an officially recognized family relationship," Chief Justice George wrote.
Before the California court’s move on Tuesday, a flurry of pro-gay marriage rulings and votes in Iowa and New England this year had appeared to reverse a trend toward banning them in the United States.
Forty-two U.S. states explicitly prohibit same-sex marriage, including 29 with constitutional amendments, according to Human Rights Campaign, a gay rights advocacy group. Several states provide for same-sex unions that grant many of the same rights as marriage. (Additional reporting by Braden Reddall, Jim Christie, Clare Baldwin, Steve Gorman, Jason Szep, David Lawsky, Dan Whitcomb and the San Francisco Newsroom, writing by Peter Henderson, Editing by Dan Whitcomb and Frances Kerry)