Why is there such a bad sterotype for white women?

All over the world the steretype about white women is that we are all s***. Yes there are white s****, but there are also s**** in every race/ethnicity. I used to have friends who were not white and they would tell me that non white guys just use white girls because they think we are all s****, yet those girls were the biggest sluts out, they were Iraqi girls by the way. This stereotype is very well known about white women. Why?