Great article, Carlyn!
I've observed, especially in marriages with children, based on statements by the many women over a period of years, that they don't like sex because it's just no fun. They aren't enjoying it because their guy isn't putting the foreplay time in, he isn't helping around the house or with the kids, and they are sick of it.
I wonder if that phase follows the phase where a man feels undesired for a lengthy period of time.
If a man's language of love is physical affection (sex) and he doesn't feel desirable, he's not feeling loved. That is heartbreaking. Especially if everything else in the marriage is working ok.
Another point is, this presents a weak point in a relationship, because all it takes is one woman who does find that guy desirable and let's him know it. Not saying it's right or wrong. Just saying it happens.