Yesterday, Jonathan Chait at the New Republic posted an article that expands on this theme brilliantly, also mentioning the NRO editorial:
Dismissing the argument that marriage might foster more stable gay relationships, the magazine's editors replied curtly, "[T]hese do not strike us as important governmental goals." There's a word for social policy that disregards the welfare of one class of citizens: discrimination.
Some hard-core conservatives are willing to openly discriminate like this, but most people aren't, which is why public opinion is warming to gay marriage. Most opposition arises from simple discomfort. When I first started hearing about gay marriage, I didn't oppose it, but it seemed sort of strange and radical--and only after several years did I realize I supported it.
The line "I believe that marriage should be between a man and a woman" is an expression of that sensibility--a reflection of unease rather than principle. As people face up to the fact that opposing gay marriage means disregarding the happiness of the people most directly (or even solely) affected by it, most of us come around. Good ideas don't always defeat bad ideas, but they usually, over time, defeat non-ideas.