Women have different sexual desires that aren't met by the simple act of fucking, so just watching two people fucking is far less gratifying. For them, sexual desire is much more bound up with psychology and the unfolding of a relationship -- especially with a handsome, powerful, dangerous person. Power and danger are to women what big tits are to men, hence the thriving market in vampire/werewolf/billionaire/pirate/surgeon erotica which does play into women's desires in ways most visual porn does not.