I am afraid that it is you who missed the point here. "Explicit" lyrics was not the point - I argued that perception of music is often filtered through racist and sexist stereotypes. So it is not the theme of "sex and violence" itself but the form of its representation.
Using your own example, the depiction of killing in Johnny Cash's tune is rather tame comparing to those in gangsta rap - and that difference correspond to the stereotypes of blacks as being more "brutish" than whites. In other words, depictions of killing by whites or in "white" song" is sanitized of all gory details and followed by a "civilized" remorse, while the depiction of killing by blacks and in "black" song is full of gory details and expletives, followed by "barbaric" gloating.
This is precisely what is so appealing in gangsta rap - it is white or middle class kids using blacks to do something they would not dare doing in their own name - piss of their parents and authority figures. It is so obvious that is pathetic that seemingly intelligent people cannot see it.
Wojtek