The theory (or law if you want) of Parental Investment dictates that as Women expend greater resources in the rearing of children then Men do, Women will be inclined to be highly selective OF mates. This suggest that competition for females will be greatest among the males of our species and as such the Males will have higher libido to compensate. I think women can be just as sexual as men can but thats more the exception proving the rule. Throughout human history it has been the menfolk who have really called the shots relationship-wise and any significant reversal of this is mostly a modern one eased into transition with the help of Legislature, and technology.