Depressing finding, but not completely shocking. When you're single, you've got more incentive to stay fit. When you're with someone for a long time, there's more comfort in knowing you're not out on the market and you can let yourself go a little. :)
I can also see having a family contributing to the weight gain as well. There's just so much less time to actually hit the gym on a consistent basis when you're a woman taking care of small children.