According to The New York Times:
When France created its system of civil unions in 1999, it was heralded as a revolution in gay rights, a relationship almost like marriage, but not quite. No one, though, anticipated how many couples would make use of the new law. Nor was it predicted that by 2009, the overwhelming majority of civil unions would be between straight couples.
The numbers are surprising:
In 2000, just one year after the passage of the law, more than 75 percent of civil unions were signed between heterosexual couples. That trend has only strengthened since then: of the 173,045 civil unions signed in 2009, 95 percent were between heterosexual couples.
Whatever their reasons, and they vary widely, French couples are increasingly shunning traditional marriages and opting instead for civil unions, to the point that there are now two civil unions for every three marriages.
It seems to me that conservatives in America should embrace marriage equality. If not, civil unions will be the alternative and it is unlikely that they will remain popular in large metropolitan areas with only gay people.