You still get some models in the old "ladies" style with 2 thin parallel tubes, but many step through frames have a single fatter Al tube, and are as structurally sound as any other single tube design. The riders who like this style do not hammer their bikes, and the frames are quite stiff enough for the forces applied. They are heavier than more efficient designs, but with Al, the extra material you need does not carry a big weight penalty.
Some women still cycle in skirts, at least in parts of the world where cycling is an everyday method of transport.
If you need to ride 3 miles into a small, flat city and need to look smart for work, then you need a different style of bike to a weekend racer.