Having attended an all-women college, living in the dorms the entire time, I realize that those were five years spent away from normal men. Aside from standard social divisions such as race, age, sexuality, etc., the main split between us students was between nursing majors and everybody else. By no means were all nursing majors vapid, shallow and dependent - just like not all non-nursing majors were deep, analytical and independent. When someone who didn't major in nursing was referred to as a "nursing major," the reference of her mind was understood.
And, aside from a few faculty members, security guards, and boyfriends of friends, men were not a social group in my mind. Because I only knew them as individuals, I thought of them as individuals rather than as representatives of a gender.
So when I graduated and came back to reality, I was extremely hesitant in thinking along the lines of "men this" and "men that." When I'd read/hear women complain that men take up too much space, I didn't want to believe it. The main division, in my mind, was still between nursing majors and everybody else, devoid of gender...alright not entirely. It's saddening to see a woman fulfill the "nursing major" stereotype because she can't imagine her own independence and value.
And now, after two years of almost daily public transportation, it's much more difficult to question women who claim that men take up too much space. About two-thirds of the people who cram me against the bus/train wall, rub up against me unnecessarily or push me into the aisle are men - all of the people who intentionally do these things are men (how do I know it's intentional? They look right at me while doing it and the women apologize).
This is probably not a natural, biologically-based behavior - at least not any more than the vapidity of many women is based in biology. Granted, I don't know the backgrounds of most of the people who act along these stereotypes, but I'd like to believe that these behaviors are nurtured socially.
In essence, a lot of people make it rather difficult to not categorize them along stereotypes.