Connecticut has moved to become the second state in the US to allow same-sex civil unions - and the first to do so without orders from a court.
Protesters tried to stop the House passing the bill
Its House of Representatives passed a bill on Wednesday that would grant hundreds of rights to gay couples.
But an amendment to the bill specifically defined marriage as being the union of a man and a woman.
Meanwhile, Oregon's Supreme Court nullified nearly 3,000 gay marriage licences issued last year.
The judges ruled that gay marriages contravened state matrimonial law. However, Governor Ted Kulongoski said he would now support a new law allowing gay couples to form civil unions to give them many of the rights of marriage.
Gay marriage has become one of the most heated issues in the US, where voters in 11 states rejected it last year.
Advocates on both sides of the issue expressed mixed reaction about the Connecticut bill, which the state's Republican governor is expected to sign into law.
State Senator Andrew McDonald, a supporter, called it "an undeniable victory of unprecedented proportions", the New York Times reported.
But Mary Bonauto, a lawyer who fought successfully for same-sex marriages in Massachusetts, expressed disappointment.
She said the amendment defining marriage as being between a man and a woman "puts into law the second-class status of gay and lesbian families in Connecticut".
"That is a very bitter pill to swallow," she told the Associated Press news agency.
The Connecticut Catholic Conference opposed the bill.
Its executive director, Marie T Hilliard, called it "a defeat that undermines marriage for all of society," the Washington Post reported.
San Francisco was the first US city to permit gay marriage
The House bill must now return to the Senate for approval and then be signed by Governor M Jodi Rell, who had pushed for the heterosexual definition of marriage to be added to it.
Vermont is the only other US state to allow same-sex civil unions.
Courts in a number of other states, including Massachusetts and California, have said bans on same sex-marriage are unconstitutional.
President George W Bush voiced support last year for a constitutional amendment outlawing same-sex marriage, but has said little about it since his re-election in November.
He mentioned the subject in passing during his State of the Union address in February.