The only reason bike were made with step thru frames were to accomodate a skirt. If you think about male anatomy having a horizontal bar 6 inches under the front of the seat makes no sense.

Except the triangle formed by that horizontal bar results in a MUCH stronger frame than the open "V" of a girl's bicycle.

Comparing similar bicycle frames, a girl's bicycle won't stand up to jumping or even rough riding nearly as well as a boy's bicycle.