If the court agrees, Massachusetts could become the first state in the nation to sanction same-sex marriage.
A similar lawsuit in Vermont in 2000 resulted in that state creating civil unions, which are not equal to marriage but carry many of the benefits of marriage. More than 4,000 couples have gone to Vermont for civil unions, but those are not recognized outside of Vermont.
In Hawaii and Alaska, legislatures passed constitutional amendments limiting marriage to men and women after courts there approved same-sex marriage.
A similar move was likely in Massachusetts if the court rules in favor of the couples.
The seven couples, four of which are raising children, have been in committed relationships from 6 to 32 years, yet are denied hundreds of benefits provided by the institution of marriage, according to New England's Gay & Lesbian Advocates & Defenders.
GLAD lawyer Mary Bonauto said banning same-sex marriage is a form of discrimination based on gender. She noted that for many years most U.S. states had bans against interracial marriage.
Opponents, including Attorney General Tom Reilly, contend there "is no constitutional right for same-sex couples to marry" in Massachusetts.
The couples, after being denied licenses to marry by local authorities, filed their suit in April 2001.
They are appealing a ruling last year by Superior Court Judge Thomas Connolly who ruled the issue should be decided by the Legislature, not the courts.
The attorney general's office agreed that "such a far-reaching change" in state law belongs to the Legislature.
An opponent of same-sex marriage, Ronald Crews of the Massachusetts Family Institute, said the state's highest court is facing a "critical" decision.
Asked by justices why Massachusetts should become the first state to legalize same-sex marriage, lawyers for the plaintiffs said because, "It's the right thing to do."
While Catholic, Protestant fundamentalists and Orthodox Jewish organizations said unions between homosexuals should not be sanctioned, the Unitarian Universalist Association supported legalization.
GLAD said it expected a decision sometime in the summer.