HARTFORD, Conn. — The idea that civil unions could be a satisfying but less contentious substitute to gay marriage was knocked down Friday by Connecticut's Supreme Court.

There is no substitute, the justices ruled 4-3 as they made the state the nation's third to allow same-sex weddings.

The ruling might not have been as earthshaking as the one in Massachusetts that allowed gay marriage for the first time in the U.S., or the one in California that made it legal on the other side of the country and in the nation's most populous state. But it cut into the view that there is some solid middle ground on an issue that has inflamed passions on both sides.

"It's another court saying that separate but equal is not OK," said Edward Stein, a professor at the Cardozo School of Law in New York City. "As state courts start to say this ... gradually, over time, there might be a consensus that emerges."

Same-sex weddings are expected to begin in Connecticut in less than a month. Out-of-staters will be eligible, but few other states are likely to recognize the unions.

The vast majority of states ban gay marriage, but some have passed laws that recognize same-sex unions in some way. The civil unions approved by Connecticut's General Assembly in 2005 are identical to marriages in virtually every respect except the name.

Connecticut opponents of same-sex marriage fought against the civil unions law but were even more outraged by Friday's ruling, the first in which civil unions were rejected as an alternative.

"Even the legislature, as liberal as ours, decided that marriage is between a man and a woman," said Peter Wolfgang, executive director of the Family Institute of Connecticut. "This is about our right to govern ourselves. It is bigger than gay marriage."

Californians will vote next month on a ballot measure that would reinstate that state's gay-marriage ban, but Connecticut's governor and attorney general said there is little chance of a similar challenge to Friday's ruling.

"The Supreme Court has spoken," said Gov. M. Jodi Rell, a Republican who opposes same-sex marriage. "I do not believe their voice reflects the majority of the people of Connecticut. However, I am also firmly convinced that attempts to reverse this decision — either legislatively or by amending the state Constitution — will not meet with success."

Evan Wolfson, head of the group Freedom to Marry, said people watching Connecticut will come to realize that legalizing gay marriage will not harm anyone.

"Couples in Canada, California, Massachusetts, South Africa and Spain are all doing better; they're happy, they're celebrating," he said, referring to the nations and states where same-sex couples can marry. "Meanwhile, the gays didn't use up all the marriage licenses. There is still plenty of marriage to share."