I would say many men replace the man in the scene they are watching with the idea that it is actually them doing the fucking. When a man views two chicks getting it on he may think, hey where do I fit in? Its not easy to imagine that it is you fucking the girl when it is just tits and pussy. On this note, it is painfully obvious why drpornz likes lesbian sex. He is in fact a woman.
However, I would like to see Mrs Robinson and Lola getting it on.