I would never cheat on any woman.
I would tell her that she's getting fat, and help her lose it. If she got lazy and continued to get fatter, yes, I'd leave her.
I have a friend who used to have a fairly thin girl. Now, 2+ years later, she is such a fat pile of garbage. She is such a lazy, gluttonous (sp?), whatever Ceebs had described. I can't stand it. I just wish he had the heart, or the balls, to tell her ...