US President George Bush has called a court ruling demanding equal marriage rights for same-sex couples in Massachusetts "deeply troubling".
The issue came to court after seven gay couples filed a lawsuit
The president vowed to "to defend the sanctity of marriage" by legal means.
The only alternative to the state supreme court's ruling "will be the constitutional process", he said.
This week the court backed its earlier decision that gay couples have a constitutional right to full marriage, not just a civil union.
That first ruling by the Massachusetts Supreme Judicial Court set off a firestorm of controversy in the US.
It led the White House to promise to defend what it calls the "sanctity of marriage", defined as the union between a man and a woman.
President Bush repeated this pledge after Wednesday's ruling and suggested a change in the constitution, which would override individual state laws, may be necessary.
"If activist judges insist on redefining marriage by court order, the only alternative will be the constitutional process," he said in a statement.
The BBC's Jane Standley in New York says the Massachusetts decision is a significant milestone and follows a year which has seen more legal recognition for gay rights in the United States and its neighbour, Canada.
But the ruling came a day after the legislature in the state of Ohio gave final approval to one of the most sweeping bans on same-sex unions in the country.
Ohio became the 38th state to prohibit the unions but its law goes further by barring state agencies from giving benefits to non-married couples irrespective of sexual orientation.
Initial ruling vague
The Massachusetts court clarified its initial ruling in response to a request from the state senate about how new legal obligations to same sex couples wanting to get married must be fulfilled.
"The history of our nation has demonstrated that separate is seldom, if ever, equal," the four justices, who ruled in favour of gay marriage, wrote in the response.
The decision by the court in November, prompted by a lawsuit by seven gay couples who were refused marriage licences, had been vague in its wording.
Activists on either side of the debate argued that it did not make clear if a civil union - which confers many of the same rights as marriage but not the title - would suffice.
Now it is clear that it will not - marriage means marriage, and the state legislature has six months to change the law to make it happen, correspondents say.
Analysts say that the first same-sex ceremonies in Massachusetts could take place as early as May.
Mr Bush denounced the first ruling and reiterated the point in his State of the Union address two weeks ago, prompting speculation that he will support an amendment to the constitution to essentially override the Massachusetts ruling.