I wonder if it's possible to explain low atheist marriage rates through contingent factors? e.g. the conjunction of a low female/male ratio and a desire to marry only other atheists; a disproportionate number of out gay people in conjunction with the widespread illegality of gay marriage, etc. As for general low interest in marriage, my suspicion is that it's something like a snowball effect from the high divorce rate. People typically model their relationships after their parents', and if their parents are divorced or never married or had an unsatisfactory marriage, there's no good model there. It's probably not a coincidence that I eventually want to get married and stay that way and have a couple of kids when that's exactly what my parents did.