Interesting, as the Fount-of-Internet-Bicycle-Wisdom Sheldon Brown has much the same to say about this, except that it is bad for the wheels, not the frame.
http://sheldonbrown.com/starting.html
# The Cowboy Mount
is popular among cyclists who learned to ride on a bicycle that was too large for them. Indeed, this is the only way to get started on a bike that is seriously too tall for the rider. This dubious technique involves standing next to the bike, putting one foot on a pedal, then swinging the other leg over the saddle while the bicycle is in motion.
The cowboy mount places the rider's weight on the bicycle while it is leaning over at a sharp angle. This puts considerable lateral stress on the frame and the wheels. Bicycle wheels, in particular, are not designed to withstand serious sideways stresses, and this poor mounting technique is very hard on your wheels.