jackrabbit - I really sympathise with you, and I think my dh would agree with you too!
Have you talked with your dw about the situation - really talked, rather than rowed/accused/put pressure on her to perform?
Try and talk to her about how you feel and ask her how she feels, but without pressurising her. Having different libidos/expectations of how often you should have sex is incredibly common, even before children are involved.
How do you behave physically with her during the rest of the time? I can't bear it when dh suddenly starts to instigate cuddles because I know what it presages. Regardless of whether I'm interested or not, it's nice to be cuddled without an expectation of sex!
It's a difficult subject for any of us to talk about, and if she is feeling that she doesn't want sex she will not want to talk to you about it because she knows how important it is to you, and how much she is rejecting you.
From my experience/understanding, it does seem that sex for women when they have young children is so low down their list of priorities that most men would be shocked!
Having said all of this, your needs are important and she does need to acknowledge this. If she is playing the victim/martyr, then you/she also need to recognise this. If you are in a position to organise a cleaner/someone to do the ironing, then do this too.
Bit of a rambling answer, but I hope it helps!