Democrats and gay groups in the US have condemned President George W Bush's call for a constitutional amendment that would ban same-sex marriages.
Conservative groups want the recent gay marriages declared illegal
Mr Bush said he wanted to stop judges from changing the definition of the "most enduring human institution".
He was speaking following the recent weddings of more than 3,000 gay couples in San Francisco.
The city's mayor, Gavin Newsom, has accused Mr Bush of moving to enshrine discrimination into the constitution.
"Throughout our history, the constitution has been used to expand freedom and fight discrimination. Today, President Bush has chosen a path that runs counter to our nation's most cherished values of freedom and justice," Mr Newsom said in a statement.
Mr Bush had earlier urged Congress to approve a constitutional amendment.
"A few judges and local authorities are presuming to change the most fundamental institution of civilisation. Their actions have created confusion on an issue that requires clarity," Mr Bush said.
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.
Gay groups have reacted with outrage.
"Same-sex families pose no threat to this country or other couples," said Gary Buseck, legal director of the gay advocacy group Lambda Legal.
"The threat to families is a proposed amendment, which would write discrimination into the constitution for the first time ever," Mr Buseck said.
"Young people listening to Bush today will wonder if they can be part of the American dream, when marrying the person they love might never be part of their future."
John Kerry, the frontrunner in the Democratic contest to find a challenger to Mr Bush in November's election, said the president was seeking to divide Americans.
"I believe President Bush is wrong. All Americans should be concerned when a president who is in political trouble tries to tamper with the Constitution of the United States at the start of his re-election campaign," Mr Kerry said in a statement.
He said that although he believed marriage should be between a man and a woman, the best way to protect gay and lesbian people was through civil unions.
A constitutional amendment would be a major political event, as it would need support from three quarters of US states, as well as two-thirds of the Senate and the House of Representatives.
Mr Bush is acting following the 12 February decision by San Francisco's mayor to grant marriage licenses to same-sex couples, and a recent ruling by the Massachusetts Supreme Court, that banning gay marriages was unconstitutional.
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
On Friday, California's Attorney General Bill Lockyer will go to the state Supreme Court to ask if San Francisco's decision violated state law.
California Governor Arnold Schwarzenegger, who opposes gay marriage, has called for Mr Lockyer "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.
Massachusetts is expected to start issuing marriage licenses to gay couples from mid-May.