Because in this society woman are known to wear short things and it is okay because then men have something to stare at. But if men wear short things and are not riding a bike or going swimming people question their sexuality. It is more socially acceptable for woman to bare skin than it is for men.
Because men and women have different styles. You would consider it weird if a man decided to go out in a skirt, wouldn't you? Plus I think in European countries guys wearing shorter shorts are more common