The turning point in this social and cultural transformation occurred in 1999, with a law that created a new civil union known as the PACS, an acronym for “civil solidarity pact.” It establishes a set or rights and duties for couples governing such matters as ownership of property, taxation and inheritance.

Before the law was changed last year to allow homosexual marriage, some gay couples used these civil unions to gain formal status. However, heterosexuals have made by far the most use of them; just 5 percent of the civil unions in France involve gay couples. A similar pattern is apparent in other European countries. Nineteen of the 28 European Union nations have some sort of civil-union legislation, and the number of marriages per 1,000 people has dropped sharply — by 36 percent since 1970. In France it has dropped by more than half, to 3.7 per 1,000 in 2011 from 7.8 per 1,000 in 1970.

Moreover, almost 40 percent of children are now born to unmarried parents, compared with less than 1 in 5 in 1990, according to EU statistics. In 2010, the majority — 55 percent — of French children were born outside of marriage.

The number of heterosexual marriages in France also dropped sharply in the first five years after passage of the civil-union law. It then stabilized, but it has since resumed its decline: The number of marriages was about 20 percent lower in 2013 than a decade earlier. There are now two PACS unions for every three marriages, according to a 2013 study by Insee, the French national statistics office.