"Le Manif Pour Tous" -- or "Demonstration for Everyone" -- is scheduled for Sunday, Radio France Internationale reported. It is expected to be the largest in the French capital in many years.
Xavier Bongibault, a member of a group called Plus Gay Sans Mariage -- More Gay Without Marriage -- said the demonstration is not for the homophobic.
He said many people who are gay or support gay rights believe marriage should be reserved for heterosexuals.
"In France marriage is not a contract about love: it is a contract that creates the framework for the protection of children. I believe a child must have a mother and father," he said. "Everyone has the right to get married. I am homosexual but if I wanted to get married I could find a woman. Marriage has certain conditions, the main one of which is that it has to be between a man and woman."
Organizers say 900 buses and five trains have been chartered to bring demonstrators to Paris. At least 40 people are expected to come from London.
President Francois Hollande promised during his campaign last year to legalize same-sex marriage. But polls show support for allowing gays to marry has declined from a high of 65 percent during the contentious debate.