Some people have studied why it's so difficult for many women to do pull-ups, and their results sound convincing to me.
(Their results also remind me of the very interesting book "The Sports Gene," which Madame L will be reviewing soon.)
From this article in the NY Times Magazine a few months ago:
To do a pull-up you need "a combination of strength, low body fat and shorter stature. During training, because women have lower levels of testosterone, they typically develop less muscle than men...In addition, they can’t lose as much fat. Men can conceivably get to 4 percent body fat; women typically bottom out at more than 10 percent."
If you're interested, you should read the whole article. Anyway, it did my little heart good to see this amazing woman, Kacy Catanzaro, blow up the course in American Ninja Warrior Tuesday night: