About 2,000 opponents of same-sex marriage marched in Washington on Thursday, insisting that children will suffer if they're not raised in traditional mother-father families.
The second annual March for Marriage follows big gains over the past year for same-sex marriage, which opinion polls suggest has come to be accepted by a majority of Americans.
The march on Capitol Hill and the Supreme Court was spearheaded by the National Organization for Marriage, and included a Roman Catholic archbishop from famously gay-friendly San Francisco.
"We will not accept judgments redefining something as obviously true that it takes a man and a woman to make a marriage," said NOM president Brian Brown, who wants traditional marriage to be enshrined in the US Constitution.
"This is the beginning. We are going to be here every year and we are going to stand up for the truth and we never go away," he told AFP.
The lineup of speakers included former governors Mike Huckabee of Arkansas and Rick Santorum of Pennsylvania, as well as the leader of France's traditional marriage campaign Ludovine de La Rochere.
"Two men, even if they are married, can't take the place of a mother," de La Rochere told the crowd in slightly broken English, prompting cheers.
In a landmark ruling, the Supreme Court a year ago struck down the Defense of Marriage Act that defined marriage under federal law in explicit heterosexual terms.
Nineteen of the 50 states, as well as the District of Columbia home to the capital Washington, now recognize same-sex marriage, which opinion polls indicate is now accepted by 55 percent of Americans.