The Answer is Slavery, Slavery goes back a long ways. However the American Slave trade create the concept of White Superiority to justify slavery. So in the South you have White Women being the ideal and being seen as chaste. With Black Men being seen as sexual brutes, and Black women being temptresses. However that falls apart because White Plantation owners were raping Black Women and selling their own offspring. So you have a situation where Black Men can't protect their women, Black women are seen as sexually loose, and White women having to reconcile their perceived status with the reality.