Eskridge and Spedale, in Gay Marriage: For Better or Worse? provide the first hard numbers I have seen on how many gay and lesbian couples would get married if they could. In Denmark, though country that has had gay marriage the longest – 15 years – only 5000 people (2500 couples) out of 5.4 million people have taken advantage of the law. A tenth of one percent of the population has registered a same-sex partnership. The best scientific estimate says that about 3% of the population is exclusively homosexual. The authors found even smaller percentages of the population using registered partnerships in Sweden and Norway, which have had similar laws for about a decade.
Eskridge and Spedale then conducted a very interesting survey of Danish homosexuals about their attitudes toward marriage and the registered partnership law. They do not report numbers for all of their findings, but they do quantify two crucial findings. First, the great majority ("more than three quarters") of their respondents thought registered partnerships were the equivalent of marriage. In other words, they are not holding back from registering until the law gave homosexual unions the same name as heterosexual ones. Second, most Danish homosexuals would get married, but the great majority ("about three quarters" of the unmarried) have not found the right person to permanently commit to.
What does this mean for the American same-sex marriage debate? Scandinavia has a different culture from ours in many respects, especially with regard to marriage. Still, I think we can get a rough guess of what would happen here if homosexual marriage were legalized across the board. I think that there would be a rush of marriages at first, but it would quickly taper off. The overall same-sex marriage rate would not begin to approach the heterosexual marriage rate. I don't think that gap would ever be closed. This doesn't settle the question of whether it would be worth creating legal same-sex marriage for the benefit of that minority of homosexuals who want it. But, fear it or favor it, there is not likely to be a big wave of same-sex marriages no matter how welcoming and equal the law might be.