Wouldn’t it make more sense for the boys bicycle to have the low, sloping bar between the legs and for the girls to have the bar level to the ground?
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I think Boys bicycles were more stable.
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The design made sense when women typically used the bicycle while wearing long skirts…the sloping bar divided the skirt much lower than the level bar.
The “Diamond” frame (actually a triangle) most typical of men’s frames are much more rigid and stronger than women’s frames.
But with carbon fiber and other various new alloys offering substantially increased strength over traditional steel or aluminum, designers are coming up with all kinds of unconventional designs.
Perhaps you’re right, in time frame design will move away from traditional designs to more user-friendly ones. Right now, however, a carbon-fiber frame is still very pricey.
If you build a diamond frame in any of the new materials, it will probably be lighter than if you designed it from those same materials using a differant geometry.
Serious riders will always pay a premium for light weight and strength before convenience.
Trust me, a boy-frame bike can hurt a girl just as badly.
– Dragonblink, who found out the painful way
I’d go for a slanted crossbar even if stability wasn’t as high. Once, while carrying groceries, I made a quick stop and slid off the seat, along the crossbar, and into the handlebar pivot. It stopped hurting after the first minute or so.
Done that Cleophus 
smack right into it.
My present bike is one of these mad flashy ones with suspension, and has something in between… I have fallen a few times and yet my various bits were unharmed. Except the time when I managed to land astride the front wheel and had tyre marks on my ahem… I will post a pic later. OF THE BIKE FRAME!
I once performed a similar stunt on a girls bike. It belonged to the sister of a friend whos bike had been stolen a few weeks earlier. I rode the bike into the back of a car going about 5 mph. My butt came off the seat and my feet off the pedals at the same time. For a split second, my hands on the handle bars was my only contact with the bike. At the same moment my feet hit the pavement the boys became acquainted with the down sloping crossbar. I fell to the ground and the bike fell on top of me. Instead of a 3 to 6 inch drop to the top bar of a boys bike, it was about a 15 inch drop to the bar on the girls bike. It hurt too bad to cry. My right nut swelled up to the size of a baseball. To this day it is still purple.
To this day my reasoning for the difference in bike frames is perfectly clear. It is better to hit something close than far away. If I did the same as above on a boys bike, I don’t feel the damage would have been as severe.