When does lack of sex kill a marriage?

To the Ladies:  Ok, you've had a couple of kids and your body isn't exactly what it used to be.  Despite that, you've worked out pretty hard, done your best, and your husband has never looked at you with more desire than he is right now.  What's stopping you from actually being a woman again?  Why can't you ever feel sexy again?  And even if you think you look like an elephant, how do you not recognize the high value your man places on sex and try to make something work?

To the Gentlemen:  How long will you deal with a wife who has zero sexual interest in you?  More than that, when you crawl into bed and put your hands on her, she grabs them and thrusts them away.  You have kids with her, share a life with her, still love her, but no sex.  Ok, not no sex, maybe worse.  The two times a month that she does tolerate sex with you, she looks like she's building a shopping list in her head.

What the hell do I do?