I think you've nailed it. I'm sure there are exceptions on both sides, but for the most part sex is a function of connection for women. Which is nearly impossible to fake. So especially if the connection has not been present for quite some time - and in fact she has grown to think of you more as an additional "child" to care for - she's not "withholding" sex as men call it, but rather, she is simply no longer attracted to you as a mate. It is extremely difficult to be intimate with someone you no longer love or respect.

I also think men don't fully understand that they, TOO, need connection to have a satisfying and fulfilling sexual relationship. Can they reach orgasm? Sure. But are they present in the moment with their partner? Or just using her as a means to an end?