It's what women used to go through. He looks in the mirror, sees a gut-- he doesn't feel sexy, he can't imagine she would find him sexy, so the libido falls. In the past he could at least draw confidence from the fact that he has a good job; but now he doesn't have a "good " job, it's an okay job, one that pays the bills, but not something that you can build an identity with. And anyway she has one just as good. What's his appeal?

The woman, by contrast, might actually feel better about her deteriorating body because she's attained value in other ways; and so she figures, "we are aging together." A narcissist does not want to hear that, ever. He can still penetrate you if you become old, fat and ugly; he can't do it if he becomes old, fat and ugly.

It's a double whammy of feeling emasculated, while his narcissistic personality structure demands that he be hyper-male, ideal male. He can't attain it. So he thinks you see him as less than masculine. So he doesn't bother with you.

So the wife asks, "is he picturing himself with other women when he has sex with me?" No; he's picturing someone else with other women when he does you. It's masturbation. It's not sexy if it's his bloated body underneath Sports Illustrated's Marisa Miller.

It's what the relatively recent fascination with MILFs is all about. The fantasy isn't that your wife is with another man; the fantasy is that your wife is hot enough that another man would want her. But this fantasy has nothing to do with how hot your wife actually is, it's about your identity: I'm so great that such a woman would want me.

http://thelastpsychiatrist.com/2008/04/the_sexstarved_wife.html