This is a problem though, women get married then they feel like they don't have to "try" as hard. Then they get fat. Then their husband cheats on them. Then the girl is just fat and single with 2 kids. A far too common story. Luckily my wife is not of these, but my I can't say the same for a lot of my friends.
Women who stay thin still get cheated on. Women who gain weight often don't.
Still, your wife is lucky that she can hold on to your fidelity as long as she watches her weight. I mean, what person doesn't love to be told "I'll be here for you as long as you are thin"?