Lol, i guess the think about novels is that it all happens in the mind and when a writer writes it, it did not happen in real life at all.
Compare this with a pornographic film where women are usually degraded and treated as sex objects to satisfy a man's desire. So i guess the disapproval also stems from the actions of the sex film industry as well. So it is not solely the thought of sex that women are unhappy about. But this is just a guess from my part.