Nevertheless, I’ve collected the marriage and divorce stats from some of the Scandinavian countries that have had some sort of official union for gays, giving legal marriage rights for a significant amount of time.
The following shows the marriages per year per person in Sweden, Norway, and Denmark, from 1850 to 2006. Vertical lines mark the implementation of rights for gay couples in each country.
As can be seen, as the countries modernized, making marriage less of a tool for getting along in life, marriage does decrease on a slight downward trend. But in every case, once gay unions are okayed, the number of marriages go up per year, not down (note also, gay unions are not counted in these numbers; these are heterosexual marriages only).
Of special interest is Sweden (red line). I remember a couple years ago reading an anti-gay marriage piece where Sweden was pointed out as an example of how gays threaten heterosexual marriage. The data they were using were from a couple years before rights were given to a couple years after, basically only that negative slope right where gay unions are officially implemented. Of course the author forgot to mention or show that Sweden, for a change in pension law, had a surge of marriages in 1989 in order to act before a 1990 deadline; you can see the peak. The author knew this; by his occupation, he had to, but he hid it to make a point. This surge should cause a drop in marriage rate in subsequent years, due to a drain of marriage eligible folks, and it does for a while. But, even after that, marriages begin going up again after gays get these rights.
The next graph shows divorces per year per person for the same countries in the same time.
Again, with modernity, divorces go up, as people, particularly women, need less and less to depend on their marriage for livelihood, and unhappy marriages can more easily split up. But that trend, which some like to blame on gay marriage, is there long before any rights are given. Not only that, but after the rights are given divorces, in all cases, go down or remain somewhat stable.
For comparison, the United States, a country that has a majority of states with laws disallowing gay unions, did have a higher rate of marriage, but it also had a higher rate of divorce than all three of those countries, by the most recent data, 2005. Respect for marriage isn’t, of course, shown in the rate of marriages if you’re dissolving them faster than anyone else, right? Marriage should go up and divorce shouldn't, and that's what's seen in these countries with rights for gay unions.
In fact, Spain, has one of the lowest rates of divorce in the industrialized world and it just voted in marriage for same-sex couples. It’s divorce rate is under half all those plotted and far under that of the United States.
At pointing out these positive trends, I’ve found that some then say that the trends don’t matter (of course :-)). They then say it’s the actual magnitude of the marriage rate that shows respect for marriage, and low respect leads to gay marriage (and yet they’ll still want to call their “symptom” a cause they hope to “cure”). While it’s true it’s mainly conservative countries that have the highest marriage rates, and true they most often don’t allow gays to marry, I don’t think, in general, their societies are those to aspire towards, (e.g. Iran).
Even still, look at the countries with the highest marriage rates, from here:
1 Antigua & Barbuda
7 South Africa
What should stick out (because it's bold and red :-)) is that one of the most successful countries in that top ten list is the one that has full-fledged gay marriage.
Anyway, I’d not be so bold as to say official gay unions are shown here to strengthen the institution of marriage. But others do claim the opposite, using small segments of this same data, possibly thinking no one will look it up, and they should be called on it.
That’s not to say I can’t imagine it could, though ;-). I do think the discussions of why marriage is important to gay couples and their children probably do get some others thinking, and I think promoting the union as recognizably beneficial for families and governments regardless of the involved anatomies, can promote marriage in general. But, no, it’s not clear in these numbers, and I’ll stick with the arguments I’ve given in the past for a couple more years of data ;-).
All data taken from each country's government’s official web sites of statistics, unless otherwise stated.