President George W Bush has endorsed a constitutional amendment that would ban marriages of same-sex couples.
Many gay couples flocked to San Francisco to get married
It comes after a court in Massachusetts ruled in favour of gay weddings, and thousands of same-sex couples married in San Francisco earlier this month.
Mr Bush said he wanted to stop "activist judges" from changing the definition of the "most enduring human institution".
Correspondents say Mr Bush has seized the initiative on an important issue in presidential election year.
"If we are to prevent the meaning of marriage from being changed forever our nation must enact a constitutional amendment to protect marriage in America," said Mr Bush.
He said that while some states might want to have legal arrangements for gay people, marriage should only ever be between a man and a woman.
He urged Congress to approve the amendment.
US CONSTITUTIONAL AMENDMENTS
US constitution has 27 amendments
Amendments require passage by Congress and ratification by three-quarters of US state legislatures
Last amendment ratified in 1992, saying pay rises for representatives and senators cannot come into effect until after an election (this amendment was originally passed by Congress in 1792)
First 10 amendments collectively known as the Bill of Rights
Only one amendment has ever been repealed - Prohibition - by a subsequent amendment
This would be a major political event, as it takes three quarters of US states, as well as two-thirds of the Senate and the House of Representatives to change the constitution.
White House spokesman Scott McClellan said earlier that President Bush wanted to end "growing confusion" on the issue following events in Massachusetts and San Francisco.
"The president believes it is important to have clarity," he said. "There is widespread support in this country for protecting and defending the sanctity of marriage."
The BBC's Justin Webb says that by coming down firmly on one side the gay marriage debate, President Bush is defining it as a political issue.
Mr Bush's main Democratic opponents for the presidency do not back gay marriage, but do not support a ban either - making the president seem firm and principled and the Democrats weak and vacillating, says our correspondent.
Earlier on Tuesday, California's Attorney General Bill Lockyer said he would be asking the state Supreme Court on Friday if San Francisco's decision to allow same-sex marriages violated state law.
More than 3,000 gay couples have been married since San Francisco mayor Gavin Newsom began issuing licences on 12 February.
California Governor Arnold Schwarzenegger has called for "take immediate steps" to get a court ruling on the issue.
San Francisco officials are fighting back, and have already filed their own lawsuit with the state Supreme Court, arguing that California's prohibition on same-sex marriages is unconstitutional.
Peter Ragone, a spokesman for Mayor Newsom, said they were following "the state constitution, which explicitly outlaws discrimination of any kind".
The weddings followed a Massachusetts Supreme Court ruling last November that it was unconstitutional to ban gay couples from marriage.
That decision could result in gay weddings in Massachusetts as early as May.
Presidential spokesman Scott McClellan said the president's actions were prompted partly by the ruling in Massachusetts.
"We need to act now," he said. "The constitutional process will take time."
The amendment was submitted by Republican congresswoman Marilyn Musgrave calling for the protection of the "sanctity of marriage" between men and women.