Attorneys for seven gay and lesbian couples today argued that they should be allowed to wed and asked Massachusetts's highest court to make their state the first in the nation to sanction same-sex marriage.

The couples' attorney, Mary Bonauto, argued they are entitled to the same emotional security and legal benefits afforded to married heterosexual partners. She compared a prohibition of same-sex marriage to the ban on interracial marriage, which was struck down years ago.

"They are locked out of a precious right," said Bonauto, who is the civil rights program director for Gay & Lesbian Advocates and Defenders. "One of the most important protections of marriage is the word. It's the ultimate expression of love and commitment."

The Massachusetts attorney general's office countered that same-sex marriages should be decided by the Legislature, not the courts.

Gay and lesbian couples are entitled to domestic partnership benefits and may legally adopt children in Massachusetts. But legislators ultimately may determine that marriage should remain a union of one man and one woman because it is designed to encourage procreation, Assistant Attorney General Judith S. Yogman said.

"The optimal setting for procreation and child-rearing is a family with a parent of each sex," she said.

The Supreme Judicial Court could rule on whether gays and lesbians are entitled to marriage under the state constitution, or could choose to send the issue to the Legislature. The court also could create a parallel system of marriage-style benefits, such as Vermont's civil unions. A ruling is not expected for several months.

The case is being closely monitored by observers nationwide amid the ongoing debate over gay rights and the definition of marriage; more than two dozen groups from both sides of the debate are filing briefs with the court. A similar case in New Jersey is working its way through the courts.

Courts in Hawaii and Alaska approved gay marriage, but legislatures in those states subsequently passed constitutional amendments limiting marriage to heterosexual partners.

The seven Massachusetts couples -- including four who have children and one lesbian couple who have been together for 32 years -- filed suit against the state Department of Public Health in April 2001 after they were denied marriage licenses. Last May, a Superior Court judge dismissed their lawsuit, saying that state law does not guarantee same-sex couples a constitutional right to marry.

Bonauto argued today that nothing in state law specifically prohibits women and men from marrying a same-sex partner. Massachusetts, unlike 35 other states and the federal government, has not adopted a "defense of marriage law" defining marriage as a union between a man and woman.

Plaintiffs, from left in first row, Julie and Hillary Goodridge of Boston, Linda Davies and Gloria Bailey of Orleans, Mass., and in second row, Heidi Norton and Gina Smith of Northampton, Mass., and Gary Chalmers and Richard Linnell of Whitinsville, Mass., listen yesterday to oral arguments on their challenge to Massachusetts's ban on gay marriages.