When dealing with the vagina in this way, envision one's own penis as a blade and plunging in with intent to kill, there can actually be some mutually satisfying results obtained and I believe that even some enlightened academic texts on sexuality from the 1970s actually encourage such fantasization. It gets a little dicey when dealing with her butthole in this way though, because actual blood is far more likely to be drawn and the lines blur and the scales of mutual satisfaction tip.