Flex in a bicycle frame occurs mainly on the down tube and chainstays in a side to side motion at the bottom bracket. Because the seat tube is more verticle there is less flex in that side to side direction but the flex that is there will be less if you change the geometry to make the seat tube shorter. You will have less flex in the tubes that have the most flex to begin with, and not necessarily more flex in a longer seat post. As for the girls bike thing, they are much more flexible because the old style bike frames were not a triangle but a parallelogram with a curve. Just my observations.