"She basically becomes sarcastic and says things like "I know, I'm a terrible wife" or "well then I guess we should just get a divorce so you'll be free of me and you can find someone else". I know that she doesn't actually mean those things, it's just her way of responding when such discussions arise."

Sooner, I pulled this from your post over on romance vs. sex. (I don't know how to do the quote thingy when moving something between two threads.) Let me give you what I hear when I read statements like what your wife says. Keep in mind this is what *I* would probably mean if I said something like this. (Your W=different person )

"I'm terrified that you think I'm a terrible wife. I even think I'm a terrible wife. I'm going to say it first before you do. I'm scared that I can't fulfill your needs. I'm even going to bring the 'D' word up first in hopes that you'll reassure me that you don't want one."

If fear and a sense of inadequacy is what you're wife is experiencing, she may be using sarcasm to protect herself. Maybe she does sense how important this is and she's scared and clueless about how to address the problem.

I could easily be wrong. It's been known to happen. She could just be trying to minimize the problem through sarcasm. Try out another interpretation of her sarcasm though and see if it helps. If she is scared and you "see through" her sarcasm, empathize with that feeling and provide her with sincere reassurance, you COULD end up earning many points!