SAN FRANCISCO — California is gearing up for the "new summer of love": the legalization of same-sex marriage beginning Monday at 5:01 p.m.

Unlike the Massachusetts law, California's new law does not limit marriages to residents of the state, and many out-of-state couples are expected.

But after wedding-day celebrations subside, gay and lesbian couples from other states face sobering challenges.

Many will return home to states that explicitly reject same-sex unions. Major gay-rights groups are urging them not to rush into lawsuits demanding that their marriages be recognized. Lawyers warn they may have difficulty divorcing if things go awry.

"This is a very serious undertaking," said Richard Williams, a Chicago lawyer who has been helping local same-sex couples weigh their options.

"It may not be accepted here in Illinois, but it is legal, it is real," Williams said. "If you go to California and get married, there are potential problems and pitfalls that you have to think about."

Likely areas of contention could include inheritance, medical decision-making, health and pension benefits, and child custody.

A study by the Williams Institute at the University of California, Los Angeles, School of Law predicted last week that half of California's more than 100,000 same-sex couples would marry in the next three years, as will an additional 68,000 from out of state.

Among them are Jeffrey Dreiblatt, 47, a Web designer, and Willie Walker, 44, a legal assistant, from Brooklyn, who fell in love 15 years ago. They plan to marry in San Francisco on Aug. 8.

"When I was younger, I didn't understand the point of getting married," Dreiblatt said. "But over the years, my thinking has changed. The law in California and the implications for New York spoke to us and said, 'now is the time.' "

Many gays and lesbians are taking a wait-and-see attitude, said the Rev. Blane Ellsworth, an independent nondenominational minister in Napa.

There is still the memory of the euphoria, and letdown, in 2004, when nearly 4,000 same-sex couples stood in line to marry in San Francisco, only to have the marriages nullified by the state five months later.

Ellsworth said those he has dealt with "are pretty smart businesspeople."

"They're saying if they're going to invest in a nice service and wedding, 'I'm going to wait until it's a sure thing,' " he said.

Impossible to pass up

But for some it's an opportunity impossible to pass up. "It's something we did not want to miss," said Inga Sarda-Sorensen, of New York City, who plans a wedding in Malibu in mid-September with her partner of 10 years, Jennie Talley.

As New Yorkers, they've already gained some rights as registered domestic partners since 2004, but they view marriage as a big advance.

New Yorkers have a big advantage over most other non-California same-sex couples. New York Gov. David Paterson has stipulated that state law requires recognition of legal marriages performed elsewhere, including same-sex unions.

Most other states have statutes or constitutional amendments specifying that marriage is between a man and a woman, and denying recognition to same-sex unions. The federal government also doesn't recognize such unions.

Worried that their court victory in California could be offset by defeats elsewhere, a coalition of nine national gay-rights groups last week issued an appeal to gays and lesbians considering California marriages.

"Couples who want to should get married ... but one thing couples shouldn't do is just sue the federal government or, if they are from other states, go sue their home state. ...

"Bad rulings will make it much more difficult for us to win marriage, and will certainly make it take much longer."

Worried about legal chaos, attorneys general from several states asked California to postpone the start of same-sex marriage until after Nov. 4 voting on a state ballot measure that could ban such unions. The request was denied.

Debbie Hunt, a Houston lawyer with many gay clients, worries that same-sex couples from Texas will marry in California and return home with false assumptions that they have marital protections in a state that has banned same-sex unions.

"My job is to educate, let our clients make the ultimate decision," Hunt said. "But if they ask 'Should we do it?' my answer is: 'If you don't have to, please don't.' "

Couple dissuaded

Hunt's advice helped dissuade Regina Cassanova and Jheri Dupart not to venture to California, though the partners of more than 10 years had been thrilled by the ruling there.

"Even though we want the legal protection, maybe this isn't the right time for us," said Cassanova, 27.

Planning a California wedding — date to be determined by availability of discount airfare — are Bonnie Aspen, 55, and Willow Williams, 48, of Spokane. They've been together 29 years, and they registered last year as domestic partners when Washington adopted that policy.

While the state has not legalized same-sex marriages, this past week a law went into effect that adds more rights of marriage to registered domestic partners.

Even if the marriage has no immediate practical effect, they yearn for it, Aspen said. "We don't expect to come back and — boom — Washington says you're married," she said.

"Getting a marriage license is one more step to help our state and the rest of the country move forward. In another 20 years this country will be very embarrassed by what it put some of its people through."