By Peter Henderson and Amanda Beck
SAN FRANCISCO, June 16 (Reuters) - California performed its first legally recognized same-sex weddings on Monday and opened its doors to gay and lesbian couples from around the country, a move likely to challenge other states that define marriage as between a man and a woman.
Octogenarians Del Martin and Phyllis Lyon took the first vows in San Francisco's ornate City Hall in a ceremony presided over by Mayor Gavin Newsom, who said California was leading a trend that would sweep the nation.
Acceptance of gay marriage has grown in the United States but most states outlaw it and, overall, Americans prefer to give marriage-like rights to homosexuals under a different name. In a November election, Californians have the option to end same-sex marriage, making the next few months a crucial test of whether gays and lesbians can convince fellow citizens that there is no going back on marriage rights for all.
Martin and Lyon, who have been together for more than 50 years, were also the first to be married in 2004, during the "Winter of Love" when about 4,000 same-sex couples exchanged vows. Courts overturned those unions but last month the state Supreme Court struck down a ban on homosexual marriage, with that ruling going into effect late on Monday.
"When we first got together, we weren't really thinking about getting married," Lyon said to a small crowd that laughed, cheered and threw red rose petals.
Southern Californian couple Robin Tyler and Diane Olson, two of the plaintiffs in the state Supreme Court battle, exchanged vows in Beverly Hills, where for years they had petitioned unsuccessfully for a marriage license.
"Here is the final ending to our beautiful story – 'And they lived happily ever after'," Tyler said. Nearby, a supporter waived a placard that said "Finally."
Still, opponents aim to fight back in November and change the state constitution to define marriage as between a man and a woman. Conservative Christians have vowed to get voters to the polls.
California is the second state, after Massachusetts, to marry same-sex couples but it is the first ready to grant licenses to couples from any state. Gay marriage is rejected by 45 states, although New York will honor California unions.
"If marriages performed outside of New York are going to be recognized, I'm sure it won't be too long before (gay) New Yorkers will be able to be married in their own state. So already it is having an impact that crosses to the Atlantic Coast," said Star Trek's Mr. Sulu -- actor George Takei -- who plans to wed longtime partner Brad Altman later this year.
"We are boldly going where no one has gone before," he said, jokingly echoing the opening of the TV series.
Many countries allow domestic partnerships, although a relative few recognize gay marriage, including Belgium, Canada, Spain and the Netherlands.
Around California, the most populous U.S. state with more than 36 million people, a few marriage offices started ceremonies after 5 p.m. (8 p.m. EDT/0000 GMT) and planned to work into the evening. Hundreds of volunteers have been deputized to marry couples in tents and on courthouse lawns.
They may face opponents such as protesters in San Francisco waving signs reading "Homo Sex is Sin" and similar warnings.
Tony Perkins, president of the conservative Family Research Council, predicted "months of social chaos that could wreak havoc on every state in America."
He said California's new policy "threatens to undo thousands of years of natural marriage." Less than a third of Americans responding to a recent CBS poll said gay marriage should be legal, although the trend is toward growing acceptance. More than a third opposed gay marriage.
University of Southern California law professor David Cruz predicted the practicalities of married gay couples moving from California to other states would spark change.
"People's attitudes are already changing, and what will change public opinion in favor of same-sex marriages further is knowing same-sex couples and seeing them live their lives like other married couples," he said.