It's interesting to me that it does seem to be social norm that women feel sex is an intimate duty or obligation for men --men need it, they must have it and women must give it to keep them happy. And society seems to endorse it. For women, in their normal life cycle, reproductive factors of child bearing and hormonal changes might greatly influence their sex drive over time, and my belief is that this is actually normal. Does that mean there is something wrong? Not necessarily, but I do believe social pressure makes women fear being left for a younger woman and from that fear comes obligation.
Now that women are increasingly able to support themselves in the labor market, I expect changes will be happening in norms around sexual politics. You can also see that big changes in gender/parental expectations have already changed because of how female roles have changed in the workplace. (Today, more men than ever are stay-at-home moms perhaps because the female has greater earning potential.) I believe that a healthier approach is if either partner experiences a period of low sex drive, it can be discussed without blame/shame/obligation, as a part of their life cycle rather than in spite of it. I believe that this is a healthier, more flexible platform on which a long-term partnership can find an honest balance.