[T]he French pact civil de solidarité – a kind of civil union introduced in 1999/2000, [was enacted] largely as an alternative to gay marriage. But the pacs has had very interesting consequences for straight couples (95% of couples with pacs are straight), as this chart shows.
The growth of the pacs’ popularity over its first decade is striking. There are now two pacs for every three marriages. Interestingly, this is because of both a significant decline in marriage, and a significant increase in the overall number of people willing to engage in some kind of state-sanctioned relationship.
Score one for slippery-slope arguments, or chalk it up to selection bias? More from Henry Farrell at the link. (And see the first comment, by John Quiggin.)