It is our biology and how the human race has evolved. women carry babies, and raise them, and therefore sex has a much heavier consequences for women than men. men, however, are biologically designed to spread our seed to propogate the entire species, so the way we have evolved is optimal to do that. we get turned on in a snap so the species can survive, we are turned on by physical features because we propogate the species by choosing a mate with optimal genes based on appearancce, and we jump into sex whenever we can becasue our developed psychology tells us it is right. women are more reluctant because not only are the consequences carrying a baby for 9 months, they have to raise the baby and know they are going to need help if they do so, so want to be sure the guy they are having sex with is good enough for that job. also, society judges women who are loose with sex, becuase of developed gender roles in contemporary society. its very complicated, and most men/women just judge the way each others' minds and bodies work when it comes to sex, but it's all about the survival of the species. we may never fully understand each other as genders, but if we hadn't adapted, developed, and evolved to be EXACTLY the way we are (both men AND women), the species wouldn't have been able to survive the way it has.