> Couldn't it possibly be natural for women to be fatter than men ?
Women bear children - pregnancy and lactation might demand greater fat
reserves than needed for men.  What is the natural fatness for men ?

This is correct.  The ideal fat percentage for men is 15%; for women it's 22% 
 Male athletes can have as little as 5% fat.  Women athletes can go as low as 
12-15%  But if women's fat percentage falls below about 13%, they stop 
menstruating. Many women runners do not menstruate.  Estrogen is stored in 
body fat, and if there's not enough body fat, the woman's estrogen falls too 
low for menstruation.

In fact, there is a theory that the reason American women have such a hard 
time at menopause is because they are overfat and therefore 
over-estrogenized, so when the estrogen suddenly plummets at menopause, the 
drop is more drastic than it is for a woman of ideal fat percent.

Re pregnancy and lactation... You are again right.  When women are pregnant, 
the hormones released cause their bodies to store fat.  Lactation is 
extremely calorie intensive (2000-3000 calories a day, as I recall), and if 
women didn't have either stored body fat or access to unlimited amounts of 
food, they would run into problems.

     - Sheryl