What I don't get, from having just found this thread, is how people feel they're in a position to judge why other women don't have sex with their partners after childbirth. Instead of asking 'why', we're told that if there isn't some pathology or family violence involved then it's because we are perversely 'holding out'.
It assumes that many women are asexual, as another poster suggested, instead of experiencing some other compelling impediment to a healthy intimate life.
In a relationship that's going down the tube and there has been very little sex after children, you'll probably find there are other reasons, far more damning than not having sex that the relationship is in dire straits.
I think you'll also find that sex is one of the last things to suffer in a relationship - people continue to have sex with each other long after the functionality of the relationship is compromised. I can think of a few relationships where separated couples end up DTD - not a very good litmus test for relationship health!