Massachusetts lawmakers must give gay and lesbian couples full and equal marriage rights, the state's highest court ruled Wednesday.

With its decision to back marriage and not the concept of civil unions, the court set the stage for the nation's first same-sex marriages (search) to take place beginning in mid-May. Plus, the court added fresh fuel to a debate that has split politicians, churches and families around the country.

The court's action clarified what it meant in November when it ruled that the state's marriage laws were unconstitutional. Massachusetts lawmakers wanted to know if the court would have accepted civil unions, a legal designation used by Vermont that conveys many of the benefits of marriage to same-sex couples.

"The history of our nation has demonstrated that separate is seldom, if ever, equal," the four justices who ruled in favor of gay marriage wrote in the advisory opinion. The bill that would allow for civil unions, but falls short of marriage, makes for "unconstitutional, inferior, and discriminatory status for same-sex couples."

Paul Martinek, editor of Lawyers Weekly USA, said that the blunt opinion erases any confusion.

"The fat lady has sung and she's singing the wedding march," Martinek said. "It's clear from reading the majority opinion that there's no basis on which the (court) will OK anything other than marriage."

The much-anticipated opinion sets the stage for next Wednesday's constitutional convention, where the Massachusetts Legislature will consider an amendment backed by Republican Gov. Mitt Romney (search) that would legally define marriage as a union between one man and one woman.

"We've heard from the court, but not from the people," Romney said in a statement. "The people of Massachusetts should not be excluded from a decision as fundamental to our society as the definition of marriage."

Without the court's opinion, Senate President Robert Travaglini (search) had said the vote would be delayed.

"I want to have everyone stay in an objective and calm state as we plan and define what's the appropriate way to proceed," Travaglini said.

The soonest a constitutional amendment could end up on the ballot would be 2006, meaning that until then the high court's decision will be Massachusetts law no matter what is decided at the constitutional convention.

Gay couples could get married in Massachusetts as soon as May, the deadline set by the court last fall.

"We're going to have to start looking for a band," said Ed Balmelli, who put down a deposit for a wedding after the opinion.

Conservative leaders said they were not surprised by the advisory opinion, and vowed to redouble their efforts to pass the constitutional amendment.

The White House immediately denounced the decision, calling it "deeply troubling" and vowing to pursue legislation or even a federal constitutional amendment to protect the traditional definition of marriage.

"Activist judges continue to seek to redefine marriage by court order without regard for the will of the people," said presidential spokesman Scott McClellan, adding that the president is "firmly committed to protecting and defending" marriage as only between a man and a woman.

And legislators were prepared to vote on a proposed amendment to the state constitution that would seek to make the court's ruling moot by defining marriage as a union between one man and one woman — thus expressly making same-sex marriages illegal in Massachusetts.

"This now puts the pressure back on the Legislature to do their job to protect and defend marriage for the citizens of the state to allow them to vote," said Ron Crews, president of the Massachusetts Family Institute.

Church leaders in the heavily Roman Catholic state also pressed their parishioners to oppose efforts to allow gays to marry.

Mary Bonauto (search), an attorney who represented the seven couples who filed the lawsuit, said she anticipated a fierce battle, saying that "no matter what you think about the court's decision, it's always wrong to change the constitution to write discrimination into it."

When it was issued in November, the 4-3 ruling set off a firestorm of protest across the country among politicians, religious leaders and others opposed to providing landmark rights for gay couples to marry.

What the case represented, both sides agree, was a significant new milestone in a year that has seen broad new recognitions of gay rights in America, Canada and abroad, including a June U.S. Supreme Court decision striking a Texas ban on gay sex.

Legal experts, however, said that the long-awaited decision, while clearly stating that it is unconstitutional to bar gay couples from marriage, gave ambiguous instructions to the state Legislature.

Lawmakers remained uncertain if civil unions went far enough to live up to the court's ruling — or if actual marriages were required.

When a similar decision was issued in Vermont in 1999, the court told the Legislature that it could allow gay couples to marry or create a parallel institution that conveys all the state rights and benefits of marriage. The Legislature chose the second route, leading to the approval of civil unions in that state.

The Massachusetts decision made no mention of an alternative solution, but instead pointed to a recent decision in Ontario, Canada, that changed the common law definition of marriage to include same-sex couples and led to the issuance of marriage licenses there.

The state "has failed to identify any constitutionally adequate reason for denying civil marriage to same-sex couples," the court wrote. "Barred access to the protections, benefits and obligations of civil marriage, a person who enters into an intimate, exclusive union with another of the same sex is arbitrarily deprived of membership in one of our community's most rewarding and cherished institutions."

The Massachusetts case began in 2001, when the seven gay couples went to their city and town halls to obtain marriage licenses. All were denied, leading them to sue the state Department of Public Health, which administers the state's marriage laws.

"The dissimilitude between the terms 'civil marriage' and 'civil union' is not innocuous; it is a considered choice of language that reflects a demonstrable assigning of same-sex, largely homosexual, couples to second-class status," the justices wrote.

A Suffolk Superior Court judge threw out the case in 2002, ruling that nothing in state law gives gay couples the right to marry. The couples immediately appealed to the Supreme Judicial Court, which heard arguments in March.

The plaintiffs argued that barring them from marrying a partner of the same sex denied them access to an intrinsic human experience and violated basic constitutional rights.

Over the past decade, Massachusetts' high court has expanded the legal parameters of family, ruling that same-sex couples can adopt children and devising child visitation right for a former partner of a lesbian.

Massachusetts has one of the highest concentrations of gay households in the country with at 1.3 percent of the total number of coupled households, according to the 2000 census. In California, 1.4 percent of the coupled households are occupied by same-sex partners. Vermont and New York also registered at 1.3 percent, while in Washington, D.C., the rate is 5.1 percent.

The Associated Press contributed to this report.