Women in general are the "weaker" sex physically, their bodies weren't designed around physical work where as men were designed to be the club bearing grunting hunters. :mrgreen:
Female bones are usually lighter and smaller, while their pelvis is wider (to help with child birth). Male bones are longer, thicker and the pelvis is more narrow. Because of that, women are more prone to stress fractures than men.
As for calcium, everyone needs it, male or female. Without it, your bones become more brittle. Although osteoporosis is less common in men than in women, men do lose bone density as they age and should take calcium as seriously as women.
On a side note, women tend to have better lower body strength than males. That isn't to say every one can leg press 900lbs (as if every man can) but on average, it is easier for them to gain lower body strength faster than a male, just as he is able to gain upper body strength faster.