For women and men you add weight because biologically you have attracted that spouse you wanted and now in the comfort zone your health and looks take a backseat. Its usually more prominent in women because of pregnancy weight gain and emotional eating with hardship of marriege. For divorce I'm assuming emotional eating also comes into play. Many women and a few men turn to food to 'fill up' that stress. Another factor is aging, at 30years plus metabolism comes declines so rapidly so even those thin people who used to burn whatever kind of food they ate also start piling up weight. Again, more drastic in women. Assuming most people get married at the average age of 27, just a few years in and your body starts working against you.
Thats why experts insist you start exercise when your younger for the sake of your older self