SAN FRANCISCO (Reuters) - A California Supreme Court ruling on Wednesday clears the way for gay marriage ceremonies that could bring a business windfall to San Francisco and other cities starting this month.

"The economic impacts will be huge," said David Paisley, a senior projects manager at Community Marketing Inc.

Estimates suggest that gay weddings will pump $684 million into the California economy in three years, said Lee Badgett, research director of the Institute on Sexual Orientation, Law & Public Policy at the University of California, Los Angeles.

"I think you're going to see tens of thousands of couples and their families descend on San Francisco," Mayor Gavin Newsom said. "People are looking at this as an opportunity to come and support their families, their partners and also the city's economy."

The Supreme Court ruled last month that it was unconstitutional and discriminatory to bar homosexuals from marrying. On Wednesday the same court denied a request to delay the June 17 start of such weddings.

The rulings open the door for California to become something like a gay version of Las Vegas, where straight couples from anywhere can get married.

California will wed any gay couple, regardless of where they live. By contrast, Massachusetts, the other U.S. state where same-sex couples can marry, only weds residents or those from a neighboring state.

'SENSE OF PRIDE'

Within five minutes of the ruling last month, San Francisco's visitors bureau sent an e-mail to thousands announcing that the city where the rainbow flag and the AIDS quilt were born was still at the forefront of gay rights, said Joe D'Alessandro, president of the San Francisco Convention & Visitors Bureau.

"Pride and this era of same-sex marriage really go hand-in-hand, because one thing that a lot of couples feel is a sense of pride in having their marriages recognized," D'Alessandro said.

The group's strategies for wooing the market include a "Marriage For All" campaign to coincide with the Gay Pride Parade, which will fill city streets with more than 1 million spectators in late June.

Headlines have already caused a spike in traffic at Gayweddings.com, which helps users find gay-friendly vendors and resources, said company president Kathryn Hamm. In May, traffic on its vendor's page was up 38 percent over last year.

Joie de Vivre, a boutique hotel company with 37 facilities in California, is planning honeymoon packages that will appeal to gay clients.

Chasing this opportunity capitalizes on two trends -- destination weddings and gay tourism, a lucrative market because same-sex couples often share two paychecks without children, leaving more disposable income for travel and luxury, Paisley said.

Another reason to push the marketing now is that gay marriage may not be legal for long. In November, California voters will decide whether to amend the state constitution to limit marriage to unions between a man and a woman.

A simple majority in favor of the amendment would neutralize the Supreme Court decision.

"Generally, fall is an off-season in the rest of the country," Hamm said. "But there may be lots of couples now who are looking at a beautiful California wedding in the autumn."