Now I really figured it out. It only needed one superficial comment from a poster.
The reason girls gain weight, is that they want to marry someone who is not superficial. But all boys who want a non-fat (or 2% fat) girl are superficial. So if they are thin, then how will they know the guy is not superficial?
So they get fat, and then only non-superficial boys will date them. And then they are assured a non-superficial guy. And they don’t even lose any dates because they’re only losing the superficial guys.