This website looks at divorce statistics for various faith groups:
"Divorce rates among conservative Christians were significantly higher than for other faith groups, and much higher than Atheists and Agnostics experience."
By myself I am statistically insignificant, but I'm a liberal atheist, and I've been married to my one and only wife for 37 years. My own take on this statistic is that your typical atheists and agnostics tend to be much more educated than your typical conservative Christian. Education level can have significant correlation to divorce rates.
I find this really interesting, since the conservative Christians are those who are yelling the loudest about gay marriage.