The decision means the United States' first same-sex marriages could take place by May. Wednesday's decision comes in response to a question from the state Senate about whether a civil union would meet the Massachusetts court's November ruling that same-sex couples were entitled to marriage rights. Civil unions are permitted in Vermont.
The high court's ruling focuses on the discrepancy in the terms "civil marriage" and "civil union," noting that the rhetoric is central to the issue.
In reference to legislation under consideration in the state Senate, Chief Justice Margaret Marshall and three other members of the state's Supreme Judicial Court wrote, "The bill's absolute prohibition of the word 'marriage' by 'spouses' who are the same sex is more than semantic.
"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 opinion read.
The court said the push to allow only civil unions "maintains an unconstitutional, inferior and discriminatory status for same-sex couples."
The opinion also asserted, "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 legislature cannot overturn Wednesday's court decision, but Democratic and Republican lawmakers will meet next week to discuss an amendment that would specifically define marriage as a union only between a man and a woman.
Such an amendment would need to pass two votes in the legislature and would be subject to a statewide referendum -- a process that could take as long as three years.
Massachusetts Gov. Mitt Romney issued a statement Wednesday saying the state's citizens should be able to voice their opinions on such a hot-button topic.
"The people of Massachusetts should not be excluded from a decision as fundamental to our society as the definition of marriage."
White House spokesman Scott McClellan responded to Wednesday's ruling by reaffirming President Bush's opposition to gay marriage.
"The president has always believed that marriage is a sacred institution between a man and a woman. He is firmly committed to protecting and defending the sanctity of marriage. Today's court ruling is deeply troubling."
In contrast to the Massachusetts court decision, Ohio's Republican-controlled legislature approved a ban on same-sex unions Tuesday. The Ohio Legislature restated its definition of marriage as a union between a man and a woman. Similar classifications exist in 37 other states.
The Ohio law, however, also would prohibit state agencies from granting civil union rights to both homosexual and heterosexual domestic partners.