You could have sistas rimming white guys but it doesn't have the same thing going for it as white girls rimming brothas. Why? A lot of how we think about black and white people is historical and goes back to the days of slavery when white people were wrongly perceived to be superior and all other 'races' inferior, with blacks at the very bottom. Hence black girls are considered (wrongly) by many already to be at the bottom. Black girls ARE sexy - white girls just can't match a sistas booty! However sistas rimming white guys just isn't considered as nasty as white girls rimming brothas.