A hypothetical young woman isn't wearing any under a shortish skirt. She goes out and is sexually assaulted. The defense know this. She is then considered to have 'asked for it' just as women used to be (I hope it is used to be) because she chose to be 'provocative' by not wearing the modesty shorts that have become a standard item for females wearing dresses and skirts.
I have a son so I was quite surprised when I heard of modesty shorts. apart from in school I can't see much of a reason for girls to wear them really.