I'm not sure if i'm adding something of weight to this conversation but what about thinking about sex as a medium? A medium as in Sex is just another human behavior that takes advantages of the usual range of ancillary features of human psychology. In a movie, you want to play around with violence, tension, release, mystery, as you do when you write music. Sex, i think, gets a bad rap. I wonder if its assumed that because sex is primal and functional it isn't a proper "Art" that behaves as other art forms do. (taking advantage of various colors and tones of the human psychology). perhaps, i'm just playing with words, but if you conceptualize sex in this way it sort of alleviates you are the task of finding evolutionary motives for each and every little sexual proclivity. just a thought?