Gay Marriages Compared to "a Pandora's Box"
They conducted a rally a day after more than 100,000 turned out across France for the cause.
The rally, organised by conservative Catholic group Civitas, was marred by accusations that journalists covering the rally and topless counter-protesters partially dressed as nuns had been roughed up by demonstrators.
French feminist writer and columnist Caroline Fourest said about a dozen topless activists from the Ukrainian-based women's movement Femen had shown up to the protest dressed as nuns and chanting "humourous" slogans.
"When they moved toward the demonstrators, some of (the protesters) ran after them, raging," Fourest said in a phone call with AFP from a police wagon, where the women had been taken for their own safety.
She claimed the women were hit by their pursuers, who also lashed out at journalists filming the scene.
Several photographers were roughed up, an AFP photographer said. The local police station said officers had made five arrests.
French Socialist party first secretary Harlem Desir denounced the aggression as "stupid".
Among the banners being held by demonstrators was a large one reading: "France needs children, not homosexuals."
The protesters included several young people wearing cassocks, a Christian clerical garment. Others waved the French flag and banners depicting the Christian cross and other emblems.
"Our objective is to wage a real battle to protect the family and child," Civitas official Alain Escada said.
He claimed gay marriage was "a Pandora's box" that would let others demand extended marriage rights, including polygamists and incestuous people.
Civitas claims 1,200 members and a network of about 100,000 supporters.
Bertrand de la Buharaye, a pensioner from Dinan in northern France who came to the rally on a Civitas chartered bus, said: "A child can not flourish normally without a father and a mother. It's against nature."
Social Affairs Minister Marisol Touraine said she respected demonstrators' concerns but that the government was not scrapping its bill, which will be debated by the national assembly on January 29.
Saturday's rallies had passed in a mainly family-friendly spirit, but police in the southeastern city of Lyon detained around 40 youths who had come to oppose the rally and police in the southwestern city of Toulouse used tear gas against a group of several hundred counter-protesters.
French President Francois Hollande's government has come under fire from Catholic groups and the right-wing opposition over the bill.
Pope Benedict XVI on Saturday called on the French church to make its voice heard on social issues.