The body's metabolism - the ability to burn off excess fat, slows down in the late twenties and early thirties, leading to weight gain in some people, and that is often the age when people marry, so the two can connect. Additionally, eating as a couple usually means proper evening meals, sometimes combined with a more sedentary lifestyle, staying in with young children etc., and this all leads to an increase in weight. Mothers have the additional problem of slackening stomach muscles which increases with each birth, responding only to exercise, for which there is litle time - see the beginning of this answer. All in all, getting older can mean getting fatter - it's all part of the process.