Under the slogan “one dad and one mom for every child” opponents of the proposal called it a “major and dangerous upheaval”.
Protests took place on the streets of Rennes, Nantes, Dijon, Bordeaux, Lyon, Marseille and Toulouse as part of the “Demonstration for All” rally organised by Catholic groups.
The aim was to persuade French lawmakers to abandon plans to pass the new law called “Marriage and Adoption for All”, which is being championed by France’s Socialist President, Francoise Hollande.
The crowds largely respected requests by organisers to wear blue, white and pink clothes and abstain from brandishing the names or colours of their own organisations or political parties, according to the France 24 news website.
“A child needs a father and a mother, he needs the paternal and the maternal side and with this bill that might not be possible any more,” said one protestor, Marthe Vignault.
“That’s the way it is and we can’t go against nature.”
Police said at least 70,000 took to the streets in Paris; there were other demonstrations in the cities of Lyon, Toulouse and Marseille.
The atmosphere was lively throughout the march, with many young people present.
Despite opposition from more than 1,000 mayors and the Catholic Church, the government approved a bill on the issue earlier this month which will be debated by parliament in January.
France already allows civil unions between same-sex couples, but extending their rights was a campaign pledge of Mr Hollande before he was elected in May.
The head of the French Council of Catholic Bishops, Cardinal Andre Vingt-Trois, recently described gay marriage as “the ultimate deceit”.
At present only married couples – not civil union partners, including opposite-sex ones – can adopt in France.
