There should be big differences in frames for size and weight if one is optimizing for performance. That is, IMHO, frames for lighter people should generally be made with lighter gauge tubes or frame walls. To this end, "girls bikes" of the same size could be made with a bit lighter wall thickness than "boy's bikes". That's not just for weight: its for comfort and shock absorption. The geometry, to me, is a second order problem. You can adjust geometry with seat height and forward/rear positioning, handlebar width and stem length, and crank length. Color is funny. A 16YO girl might want a pink bike. A gal with a little more maturity might choose something more universal and classic and less "girly". So when a woman has the means to buy an expensive bike, she may not be looking for a hot pink frame, with unicorn decals.
The discussion of color reminded me: When I was young, I had a bike in a "girls color". Then, I was a strapping (6'2", 230#) college football player manly man - Nobody mentioned the color. I'd love to have that '71 Raleigh competition back again:
