Waving red and green flags, a crowd estimated at around 74,500 by an independent media count marched from the French Senate toward the monolithic Tour Montparnasse skyscraper, chanting “Liberty, Equality, Paternity” — a play on the French motto which ends with “Fraternity”.
Many wore the distinctive cone-shaped red Phyrgian hats that are a symbol of the French republic.
Organisers, however, said they believed 600,000 had turned out against the law, which was approved by the lower house of parliament last month.
“For two years now our attempts at dialogue have gone nowhere… the street is the only place left for us to be heard,” Ludovine de la Rochere, president of the Protest for Everyone association, told journalists Sunday.
President Emmanuel Macron pledged during his 2017 election campaign to support the law despite deep resistance among rightwing opponents as well as conservative Roman Catholics and other religious groups.
They had mobilised massively in 2012 and 2013 against a move by Macron's Socialist predecessor Francois Hollande to legalise marriage for homosexual couples, with one Paris protest attracting 340,000 people according to police.
But that law did not allow lesbian couples or single women to have children via in vitro fertilisation or other medically assisted means, long a taboo in France.Many women have instead been forced to go abroad for such treatments, and French courts often refuse to recognise the second mother's maternity rights in the case of same-sex couples.
Under the proposed law, France's healthcare system would cover the cost of the procedure for all women under 43.
It would also allow children conceived with donated sperm to find out the donor's identity when they turn 18, a change from the country's strict donor anonymity protections.
If approved, the law would bring French legislation in line with other European nations including Britain, Spain, Portugal, the Netherlands, Ireland, Belgium and Scandinavian countries, which authorise medically assisted procreation for all women.
'Much less divided'
Opponents say the law deprives children of a necessary paternal figure and threatens a traditional family structure, and would open the door to legalising surrogacy, including for gay men.
Protest organisers chartered two high-speed TGV trains and around 100 buses to bring people to Paris on Sunday, and said they had distributed some three million flyers against the law in recent weeks.
“The family, with a mother and a father, is an ecosystem that needs protecting,” said Christian Kersabiec, 68, who came to the march Sunday from Vannes in Brittany, denouncing “this new society where they play the sorcerer's apprentice.”
But analysts say French society has become more accepting of non-traditional families since the gay marriage law was passed in 2013.
Last week, National Assembly lawmakers voted to make it easier for parents who have children via a surrogate mother abroad, in countries where it is legal, to have them officially recognised in France — though Macron's government has said it opposes the move.
An Ifop poll in September found that 68 percent approved IVF and other medically assisted procreation (MAP) for single women, and 65 percent for lesbian couples.
“It's hard to predict what will happen Sunday but public opinion is much less divided on MAP than on gay marriage,” Frederic Dabi, an Ifop director, told AFP.
Gerard Larcher, the rightwing president of the Senate who protested against the gay marriage law six years ago, is not expected to attend the rally Sunday.
“The people demonstrating will be doing it more out of moral conviction, a sense of duty, rather than to try to make the government back down,” said Yann Raison de Cleuziou, a sociologist who has studied the conservative Catholic movement against the family reforms.