By standard, the rule of thumb about centers of balance and gravity are the areas that usually store the most bodyfat. For women, bodyfat tends to be stored in the hips/butt/thighs, and their center is in that general area. For a man, bodyfat is stored in the stomach area, love-handles, etc. The center for your average human male is actually considered to be(depending on body-size...) 2-3 inches ABOVE the navel.
Women are considered to have better balance because we interact with the world mostly from an above-the-waist perspective. Modern society doesn't much need the full range of motion to bend over, sideways, backwards... In Judo the best examples are that when a man and woman engage in sparring practice, the woman will be able to use hip throws, and many shoulder throws against a man than a man would against a woman.
The inverse of this is also true, however, that women are ordinarily more susceptible to foot/knee/leg manipulation, like sweeps and trips.