The French National Assembly is due to debate the legalisation of same-sex marriage after months of impassioned public protests and counter-protests.
The bill, promised by President Francois Hollande during his election campaign last year, allows for same-sex marriage and adoption by gay parents.
Mr Hollande's Socialists and their allies command enough parliamentary support to see it through.
However, opponents have outnumbered supporters at recent demonstrations.
Between 340,000 and 800,000 people surged into Paris on 13 January for a rally against gay marriage, compared to between 125,000 and 400,000 who turned out on Sunday to support the bill.
The size of popular opposition and slickness of the "anti" campaign have surprised many, correspondents say.
Nonetheless, opinion polls suggest that around 55-60% of French people support gay marriage, though only about 50% approve of gay adoption.
'Equality in love'
It is expected that the legislation will reach the statue books by the middle of the year, France's AFP news agency reports.
The Socialists enjoy an outright majority in parliament and the bill is also supported by the Greens, Communists and some centrists.
In a measure of the bitterness of the public debate, Serge Dassault, CEO of the Dassault industrial group and a conservative politician, predicted in November that gay marriage would lead to the decline of the French nation, like the peoples of ancient Greece.
"We'll have a country of gays and in 10 years there'll be nobody left - that's stupid," the senator for the centre-right UMP party said.
In September, Cardinal Philippe Barbarin, the Roman Catholic archbishop of Lyon, argued that plans to redefine the concept of marriage would open the door to incest and polygamy.
Bertrand Delanoe, mayor of Paris and one of France's few openly gay politicians, retorted that the cardinal must have "freaked out".
Speaking about Sunday's rally in Paris, Mr Delanoe said: "There is a big difference between today's march and the one two weeks ago, which is that this demonstration is one of brotherhood, not of hatred.
"The majority of French people wants all couples to have equality in love and parenthood."
The debate in parliament is expected to last two weeks and marks one of France's biggest social reforms since the abolition of the death penalty in 1981.