Um, I'm a pretty staight-arrow, no-leather kind of guy and even I know that associating "top" with "penetrator" and "bottom" with "penetratee" is erroneous. I've had plenty of sex where the woman was pretty much calling all the shots - and I was inside her. I fail to see why that arrangement couldn't be formalized in the Master/slave dynamic. After all, I imagine your average BDSM advocate can be pretty creative.