Someone opined that skirts might get caught in the rear wheel. Yes, and in the days of my youth in the 1940's USofA, "girl's bikes" typically came with a net or pierced metal guard covering the upper and forward portions of the rear wheel to prevent just that.

Example: [![https://farm5.static.flickr.com/4139/4861219858_8a56c09194_o.jpg][1]][1]

edit: It is not a frame tube, but it is something "between the legs" and a difference between bikes for different genders.

  [1]: https://i.sstatic.net/DJ3vr.jpg