It is not really that people are not interested in the
topic.
How do you distinguish between an erotic movie with real sex and a
pornographic movie with a storyline? There aren't many movie-makers
who would want to cross the line and blur the distinction between
erotica and pornography. Any explicit depiction of sexual
intercourse is most likely to garner an X rating in the U.S. and an
automatic ban in many other countries. Not only would it make the
movie difficult to market -- most, if not all, X-rated movies go
straight to video -- but it'd also not look good on a serious
director's track record. That probably explains why there are so
few "serious" movies out there which contain such explicit
scenes.
And why would audience want to see an erotic movie with real sex?
They are watching it for the storyline, for the effects, for the
acting, for general entertainment, not really to satisfy their
voyeuristic desires to see people actually copulating. If they wish to
be stimulated by explicit sex scenes, there is always adult
entertainment for that purpose in the privacy of the home. Why
would they want to watch something that is "2-in-1" but usually
fails to deliver on both accounts?
