Cambridge City Clerk Margaret Drury processed more requests for marriage licenses from same-sex couples Monday than any other official in Massachusetts: more than 220. But she said she turned one couple away, two women from Florida who said they did not intend to move to Massachusetts.

A rejection like that could lead to a challenge of a 1913 state law that Gov. Mitt Romney (R) invoked to say that out-of-state couples cannot marry here, gay rights advocates and legal experts said Tuesday.

That issue is one of many legal vagaries that exist, now that Massachusetts has become the first state to grant marriage licenses to gay couples. The hundreds of same-sex couples marrying here this week will be navigating an uncharted legal landscape.

Out-of-state couples were a small minority of couples married here Monday, but clerks in four municipalities defied Romney's direction to turn them away. On Tuesday, the governor's office requested copies of all marriage licenses issued by clerks in Provincetown, Somerville, Worcester and Springfield. He has said repeatedly that marriages of out-of-state couples would be "null and void."

"I assume he's assessing his options and planning to move against clerks or against couples," said Mary Bonauto of Gay and Lesbian Advocates and Defenders. "He wants to put up a barbed-wire fence around the state."

A host of other issues could be headed for the courtroom, as same-sex couples seek to expand the scope of their rights, legal experts said.

For example, same-sex married couples will probably be able to file joint state tax returns in Massachusetts in 2005. But some may attempt to file joint federal income tax returns as well, in an attempt to challenge the 1996 Defense of Marriage Act, which defines marriage for federal purposes as the union of one man and one woman.

"They would probably be rejected, but I would take the battle to the Supreme Court on equal-protection grounds; the arguments are quite cogent," said Howard Medwed, chairman of the tax practice at the Boston firm Burns & Levinson. He said he has identified penalties in the tax code that could add up to more than $1 million for an average gay couple over their lifetimes. "I think tax will be one of the battleground areas," he said.

Inheritance tax burdens will also be significantly steeper for same-sex married couples because heterosexual couples can bequeath tax-free to their spouses, he said.

Justin Nelson, co-founder of the National Gay and Lesbian Chamber of Commerce, said same-sex married couples will be denied federal Social Security survivorship benefits upon the death of a partner.

The denial of certain federal employment benefits could also provoke litigation. Benefits provided under the Employee Retirement Income Security Act, such as insurance and retirement plans, are regulated by federal law, which would not require companies to offer such benefits to gay couples.

Peter Ebb, a partner in the employment and labor practice at the Boston law firm Ropes & Gray, said that companies might decide to scale back domestic partnership benefits currently given to unmarried gay couples. "Up until now they've been able to say we are offering this to gay couples because they can't get married. They can't say that anymore," Ebb said. "If they are challenged, legally, those programs would be very tough to sustain."

The Boston Globe reported in April that the Beth Israel Deaconess Medical Center, which currently provides benefits to the domestic partners of gay employees, will offer benefits only to married couples, beginning next year.

Adoption may also be an area of contention.

Some countries, such as China, exclude gay men and lesbians from adopting children, while certain U.S. states, such as Florida, also pose challenges, said Marla Allisan, a lawyer and social worker who founded Full Circle Adoptions, an agency in Northampton, Mass. As a result, many gay couples conceal their sexual orientation on adoption forms, which will be harder if they are legally married.

"I am recommending to our gay and lesbian families that they postpone any plans to get legally married until such point as they have completed adoptions," Allisan said.

But even with so many details to be worked out, Bonauto, who argued the marriage case on behalf of seven same-sex couples, said that another legal battle was the last thing on many couples' minds. "It's not going to be a litigation juggernaut," she said. "Although there are certainly more battles to be fought."