PARIS — Frigide Barjot has made a career of mocking the establishment of France, dressing in fluorescent pink sweaters, playing in a band called the Dead Pompidous and hosting a philosophy soirée at which she handed out T-shirts with the logo, “Kierkegaard is my homeboy.”

But Ms. Barjot, born Virginie Merle 50 years ago, has also rediscovered her religious roots, writing a book called “Confessions of a Trendy Catholic.” And she has become one of the main actors and voices in France’s fierce debate over gay marriage, adoption rights and state financing for procreation assistance. It is a debate as sincere and confused, in a way, as Ms. Barjot’s own involvement.

Despite her love of mockery and her support for the rights of gay couples, she is strongly opposed to gay marriage, and especially to the part of a proposed law that would allow married gay couples to adopt children and be recognized as their parents. On Sunday, she will help lead a large demonstration called “La Manif Pour Tous,” or a demonstration for everyone, a follow-up to an initial protest in November.

Legalizing gay marriage — “marriage for all” — was a campaign promise by President François Hollande, a Socialist, but it has proved tricky and divisive in France, which is a secular republic but remains an essentially Roman Catholic country where few go to church. He promised to enact it within a year of taking office last May, and his draft bill will go before Parliament for debate by the end of this month.