> My guess:
> Men's bikes have the bar across because it makes the frame sturdier, or something along those lines.
> Women's bikes don't in order to leave room for the skirts of the womans dress.


your guess is almost exactly on target. Back in the old days when the bike was first invented (it didn't even have pedals, you just ran your feet on the ground), they made it with a triangle shape in the center section for strength. Then women started wanting to ride bikes. Of course, they met with the usual pigheaded resistance at first because they were women and therefore incapable of doing anything that didn't involve cooking, cleaning, or changing a baby. When they finally got acceptance, they found that their skirt would ride up a bit too much on the crossbar, which would show a little too much (i.e., maybe up to their shins ;) so they redesigned womens bikes to make room for the skirt.