Couples in general tend to gain weight. Part of the reason is being in relationship/marriage usually centers around eating, drinking, watching television together, and various other (sedentary) activities.
They tend to relax their physical standards once they "have someone" because they no longer feel they have to "compete".
Having said that it's not uncommon after a breakup or divorce that these same people (change) their eating habits, start an exercise program, and participate in more evening and after work activities until they become attached again. The cycle then repeats itself.