Still, at least men don't get fatter as they get older. Oh, wait...
Ignoring the massive double standard being operated here, the reasons both sexes get fatter as they get older are the same:
1) More food. Generally people in a steady relationship eat more.
2) Less exercise. Not so much deliberate trips to the gym (but they get less frequent and intense), but all the other bits of life now get done slower or not at all.
3) Less care. Linked with 1), once you've settled you're less likely to look after your weight.
4) Age. The older you are, the less food you need to keep a constant weight. But few people reduce their food intake with age.
M