Jazz Shaw has fun watching same-sex marriage supporters explain why polygamy isn't next.
I suppose one possible explanation is that, even though there was zero value seen in the traditional definition of marriage as being the union of one man and one woman, the math underlying the equation was somehow sacrosanct. It didn’t really matter what genders filled the values of A and B in the principle of sums so long as C was still equal to two. Any other number would be an insult to… something.
A perhaps more cynical argument might be phrased as follows: Holy cow! We spent so long arguing against that slippery slope theory where letting gays marry could lead to polygamy that we’d damned well better come up with a convincing policy presentation now or we’re going to look like a bunch of self-serving asses! Find us an ethicist, stat!