Depends on if they are willing to do something about it. You have one life, I don't particularly want to be stuck around banging a fat woman I'm not attracted to physically but more importantly for me, I absolutely without question don't want a fat wife teaching my children bad eating habits.
Bringing up kids is balance between teaching them whats right and not pressuring them into being a depressed mess. Trying to teach your kids the right way to eat healthily and stay healthy and strong while your wife is teaching them the opposite is not going to turn out well.
Having been on the end of that, parents who taught be horrible eating habits, which have effected my life along with many other problems greatly. My eating habits after I became injured with bad joints became a big problem, my life would have been better if I was brought up with different eating habits, that is simply fact, I'd be happier, healthier, have less bad joints and be in less pain and less depressed.
For me its crucial, its a HUGE priority that I do my absolute best to make sure I give my kids the best chance in life at being happy and healthy, a fit healthy wife who knows how to eat and how to teach her kids to eat is crucial to that.
I wouldn't so much leave my theoretical wife for getting fat, as for if it became a problem where she was putting my kids future at risk essentially.
Everything you do in life will be easier, more fun and better if you are fit, healthy and pain free, happier and less self conscious about the way you look.