Why does society promote the idea that men sleep around more than women?

I'm not buying into the "All men are evil and all women are innocent little princesses" tripe. That world-view gets constantly drummed into our heads (just listen to a Beyonce song). You don't need a survey to prove women are just as slutty as men, just take a walk around London on a Friday or saturday night. Men and women are just as bad as each other, morals are a rare quality in people in todays world.