It seems there must be a causal link between the popularity of the union and the linguistic acceptance of "pacsé / pacser" in a language usually considered less flexible , whereas on the other hand, the fact that civil unions have no federal recognition and are only contracted by same-sex marriage makes the the linguistic change less acceptable.
Besides, Americans have a more traditional view of marriage than secular Europeans who tend to be more wary of the religious and gender-role connotations of the institution of marriage.
But even those figures cannot tell whether PACS have undermined marriage. After all, the data does not show if PACs partners would have married if the civil unions did not exist, or how many might decide to get married later anyway.
Of course, my more socially liberal view is that people, whether gay or heterosexual should have the same choice and that marriage or civil unions between two consenting adults should be granted with no restriction (well, other than they not having blood ties of course) like they are in the Netherlands, Belgium, Canada, Spain, Norway, Sweden and South-Africa. One day, we'll look back and won't believe how our societies put up with so much sexual discrimination!
There are certainly a lot of young couples in secular Europe who want the legal and emotional benefits of a committed relationship, but are put off by the religious and gender-role connotations of old-fashioned marriage. In America's relatively religious and traditionalist society, there's less such aversion; but in many states, particularly those with cumbersome and expensive divorce procedures (like New York), one could envision a fair number of couples who otherwise might have married choosing a civil union instead, if it were available. And heterosexual couples who currently feel guilty about marrying, out of solidarity with gays unable to marry, might feel obligated to pick civil unions if that were the only option available to same-sex couples.
The main problem with introducing civil unions in America would be linguistic: what adjective denotes people who are in a civil union but specifically chose that rather than marriage? Are they "partnered"? "Bonded"? "Allied"? Suggestions welcome.
Perhaps this provides grist for the mills of social conservatives (who could claim, stretching the data a bit, that gay-appeasing civil unions are undermining the sacred institution of marriage) – but it would oblige them to face up to the question of whether they should prefer gay marriage to potentially corrosive civil unions that straight couples can take advantage of too. Liberals and leftwingers don’t face nearly the same dilemma, since they can reasonably assume that those who choose civil unions over marriage have good reason for doing so (and perhaps will get married later if they want to; obviously, you can’t tell from data like this how many partners in pacs decide to get married later on).
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. While you would obviously need more finely grained data to establish this properly, the obviously intuitive interpretation of this (at least to me) is that the pacs have grown both by providing an option for people who would probably not have gotten married in the first place, and attracted a number of people who otherwise would have gotten married, but who prefer the pacs’ lower level of formality (it is much easier to cancel a pacs relationship than to get divorced)
since France introduced domestic partnerships (known as PACS) in 1999, largely to accommodate same-sex unions without having to legalise gay marriage, the number of marriages has fallen, as heterosexual couples have opted increasingly for the PACS route.