BOSTON -- When a slim majority of the Supreme Judicial Court of Massachusetts legalized gay marriage last November, the lead counsel for the seven couples involved in the case called the decision the first of many legal battles to come.

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As Massachusetts begins processing same-sex marriage licenses today, that may be the only thing advocates on both sides of the issue can agree on.

The Massachusetts decision has not only set off a blizzard of legislation aimed at putting gay marriage back in the bottle or letting it loose, it has also produced a national legal tangle that could take years to unravel.

Among the dozens of questions that will face gay couples who get legally married in coming weeks:

Will they file state taxes jointly but not their federal taxes? Could one spouse sponsor another for permanent residency in the United States? Most states do not recognize gay marriages, but will they check to see if a married couple from Massachusetts is gay after they take up residence and apply for family health insurance from one spouse's employer? How will courts in other states handle divorces, property transfers, inheritance or child support? If one spouse is disabled in another state, will the other spouse be blocked from making medical decisions on his or her behalf?

"I think the common misperception out there is that there is going to be some big, grand constitutionally based marriage decision that's going to resolve all of this patchwork quickly. That's not going to happen," said Matt Foreman, executive director of the National Gay and Lesbian Task Force.

"There is no clear legal picture. It's going to be a patchwork. Some states are going to honor same-sex marriages, some states are not, and some states are going to fall somewhere in between."

Glenn T. Stanton, senior analyst for marriage and sexuality for Focus on the Family, a main opponent of gay marriage, agrees.

"There are potentially 49 other judicial time bombs ready to go off," he said. "We're not talking about just Massachusetts. We are talking about these battles, these fights, being brought to every one of the states and that is why [some state legislatures] are pushing so hard to get constitutional amendments [to ban gay marriage]."

The one legal arena where things seem clear, at least for now, is federal law.

Under the 1996 Defense of Marriage Act signed by President Clinton, marriage is defined as a union between a man and woman and the word "spouse" refers only to a person of the opposite sex. That means same-sex couples who marry in Massachusetts will not be able to file joint federal tax returns or receive federal benefits, such as Social Security survivor benefits.

The Defense of Marriage Act also holds that no state is required to recognize same-sex marriages performed in other states. Therefore, if a married same-sex couple from Massachusetts moves to another state, it will be up to that state's legislators, attorneys general and courts to determine whether the Massachusetts marriage is recognized fully, partially or not at all.

A state might choose not to recognize such a marriage except for specific purposes, such as hospital visitation rights.

More than two-thirds of the states already have their own defense of marriage laws and have made it clear they do not intend to recognize same-sex marriages from other states. In a number of states, lawmakers and citizen movements are seeking to enshrine such laws in their state constitutions so they would be more difficult to change.

Meanwhile, lawmakers in some states have proposed legislation to legalize same-sex marriage and in at least 10 states -- including California, Washington, New York and New Jersey -- litigation similar to the Massachusetts equal-rights case that resulted in the legalization of gay marriage is underway, according to the National Gay and Lesbian Task Force.

There also are likely to be legal challenges to the constitutionality of the federal Defense of Marriage Act. Foreman expects the first lawsuits to be filed by the end of the year.

Constitutional scholars expect these challenges to say the act violates the 14th Amendment's guarantee of equal treatment for all citizens and the "full faith and credit" clause that says a judicial order or contract affirmed in one state must be recognized and enforced by the other states. They also might claim that the Constitution gives Congress no authority to legislate on marriage, therefore leaving the matter up to the states.

Laurence H. Tribe, a professor of constitutional law at Harvard Law School who supports the legalization of gay marriage, thinks the most vulnerable provision of the Defense of Marriage Act is the license it gives states to ignore same-sex marriages from other states.

"It's very doubtful, I think, that Congress has any power at all to invite other states to discriminate against same-sex marriages from Massachusetts," Tribe said.

Gay marriage advocates also can point out that the 1996 law itself, even while denying federal recognition to same-sex marriages, otherwise bans discrimination based on sexual orientation.

Nevertheless, Tribe said, "They will still have an uphill fight. The fact that Massachusetts will have same-sex couples that are legally married will give them a leg up in challenging the federal law. But it's not a case that's kind of a no-brainer. It's not like he federal Defense of Marriage act lies down and plays dead just because Massachusetts has broadened the definition."

Advocates of traditional marriage hope to short circuit lawsuits against the Defense of Marriage Act by amending the U.S. Constitution to define marriage as a union between a man and a woman. An amendment sponsored by Rep. Marilyn Musgrave, R-Colo., was debated before a House judiciary subcommittee on Thursday, and its sponsors hope it will come up for debate before the full Congress this year, though that appears increasingly unlikely.

To take effect, the amendment faces high political hurdles: It would have to be approved by two-thirds of the members in each house of Congress and ratified by three-fourths of state legislatures.

In announcing his support for the Musgrave amendment in February, President Bush called attention to the legal claims that are sure to be made against the Defense of Marriage Act and argued that a constitutional amendment was needed to protect the traditional definition of marriage from "activist courts."

Civil unions may come under legal assault, as well.

Hadley Arkes, a professor of jurisprudence at Amherst College who has argued against gay marriage, believes the Massachusetts case may invite litigation questioning the legality of civil unions for same-sex couples. Civil unions have been approved only by Vermont, but legislatures in a number of states, including Massachusetts, are considering them.

Civil unions in Vermont entitle couples to the same rights and responsibilities as marriage without invoking the term, but they provide no benefits or protections at the federal level. Also, they are not automatically recognized by other states, as marriages traditionally have been.

In Massachusetts, providing for civil unions would require a state constitutional amendment because the Supreme Judicial Court has called them an unacceptable substitute for marriage. The court said they'd create an "unconstitutional, inferior, and discriminatory status" for homosexuals.

"The next move will be to challenge the Defense of Marriage Act," Arkes said. After that, he said, gay rights activists are likely "to challenge every scheme every place where there's been a provision for civil unions. Because the court of Massachusetts sort of gave that game away."

In the months or years before such legal challenges are resolved, same-sex couples who want to marry will have to make difficult choices about how to deal with the uncertain nature of the law.

Cheryl L. Andrews, a selectman in Provincetown, Mass., is planning to marry her partner Thursday, the first day that same-sex marriages can take place after the mandatory three-day waiting period.

One thing she thought a lot about is how to handle her income tax forms. Asked whether she plans to check the married box, she answered: "You bet."

"Whether or not the federal government is going to find that they have the free time and money to spend taking gay couples to court to challenge their tax returns is going to be a rather awkward situation for them," Andrews said. "I'm going to be hard-pressed as a married woman to lie and perjure myself on a federal document and say I'm single."