This dearth of early adopters is not peculiar to America. Research conducted in 2004 by Gunnar Anderson, a professor of demography at Sweden’s Stockholm University, seems to confirm the trend. Anderson looked at legal partnerships in both Norway and Sweden and found that in Norway, which legalized civil unions in 1993, only 1,300 homosexual couples registered in the first eight years, compared with 190,000 heterosexual marriages; in Sweden, between initial passage in 1995 and a review in 2002, 1,526 legal partnerships were registered, compared with 280,000 heterosexual marriages. In the Netherlands, gay marriage is actually declining in popularity: 2,500 gay couples married in 2001 — the year it was legalized — and that number dropped to 1,800 in 2002, 1,200 in 2004, and 1,100 in 2005. In 2009, the last year for which figures are available, less than 2 percent of marriages in the Netherlands were between same-sex couples.

Controlling for the ratio of homosexuals to heterosexuals does little to explain the enthusiasm gap. For rates to be similar, we would have to pretend that only 0.5 percent of the population of Sweden, 0.7 percent of the population of Norway, and less than 2 percent of the population of Holland is gay. In fact, the numbers tend closer to an average of 4 percent, which suggests that heterosexual couples are up to eight times more interested in registering their relationships than homosexual couples. It is, of course, possible that the estimated number of homosexuals is wrong, but, if anything, gay-rights groups tend to argue that the projected numbers are too low, and statistics show that the numbers of self-identified gay citizens are going up in every Western country.