"Men's bikes have the bar across because it makes the frame sturdier..."
>>Right.
"Women's bikes don't in order to leave room for the skirts of the womans dress."
>>Mostly so she can step through instead of swinging the leg over the saddle and rear wheel. Very unladylike, especially if she wasn't wearing any drawers (most women didn't until after WWI).

"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."
>>Early pedal bikes were "boneshaker" type with pedals on the front wheel like a child's trike today. To go faster the front wheel was made bigger and bigger clear up to 60", all anyone could straddle. Women wearing skirts on these "ordinary" or "pennyfarthing" bikes was out, though side-saddle designs were tried. Starley introduced the "safety" bike with wheels of about the same diameter, with pedals and chain drive to the rear like bikes today.

"Even mountain bikes have a discreet, ladylike inch or two lower on the crossbar, just to show it's a girl's bike."
>>It's because the canted top tube allows more clearance to avoid hitting the tube with one's vitals, plus a shorter seat tube makes for a stiffer frame.

"Hehe. I don't know just how much difference it would make on the structural integrity of the standard diamond frame --"
>>The standard diamond frame doesn't have any structural integrity (triangulation) in the front part. Its strength is dependent on the strength of the four corner joints.

"I reason that when buying and teaching their children to first ride a bicycle, many parents get something that is near adult-size, knowing that the kids are bound to "grow into" their bikes. But hey... that means there's a period of several years when the bike isn't really *safe* for the kid to ride it! Salespeople know that when a rider stands stationary and upright with both feet on the ground, there's supposed to be some 2-4 inches leeway between the top tube crossbar and the rider's more tender parts; it also helps a person to brake with his feet on the ground in an emergency."
>>If there are stays going on back to the rear dropout from the "top" tube of a drop frame it's almost as strong and stiff as a diamond. "Mixte" frames were popular for years in Europe, and many Peugeot mixtes were sent to US. They have two skinny top tubes that run from the head tube straight back to the rear dropouts. But having one larger tube between the head and seat tubes is stiffer and lighter weight.

>>The sizing should be to use the shortest seat tube that you can get a seatpost long enough to have the saddle where you need it. Pro racers don't use the largest frames they can straddle. Larger frames of the same tubing aren't as stiff. But for touring we usually don't want quite as much stiffness anyway.