So-called girls’ bikes were designed for modesty. Women and girls commonly wore skirts instead of pants until relatively recently.
This being said – bikes that are designed for men do not fit women very well. The ratio of top tube length to down tube length, in particular, is wrong for women because they generally have longer legs in proportion to their total height. Handlebars tend to be too wide, brake levers don’t fit their smaller hands, and so forth.
Engineer Georgena Terry grew disgusted with bicycle sexism and started her own company. She began designing bikes specifically for women. Most have a horizontal top tube, but the frame design is set up for a woman’s proportions. You can view here wares here:
http://www.terrybicycles.com
Petite women have the most to gain from Terry’s designs. A woman who is taller can get by with a bike made primarily for men by swapping out some parts and getting a good fit.