I agree with her, but it goes both ways, I expect my dp to stay fit and make an effort with his appearance, and sex is the glue that holds relationships together, it doesn't complete them, but it keeps it healthy and happy
I don't agree that men should be off out having affairs, but genrally it's because the wife has becomes so bloody wrapped up in being a mum, that she forgets why she fell for the person she is with.
(And the caveat of course is any abusive marriages)