When I was younger I used to think having sex just for the sake of pleasure, or god forbid for money, made you a "dirty" immoral person. Why did I think this way? Because I was raised catholic. But this type of attitude is not limited to one religion or religion in general. Our society is obsessed with sex but at the same time condemns most types of sex as "evil". Casual sex is glamorized in the media but also gets you labeled as a slut (at least if your a woman).
But where does this strange attitude toward sex come from? In the time before birth control and paternity tests there was actually a practical reason for telling young women that sex before marriage and sleeping around was immoral. Women, since they were not allowed to work, could not support a child without a husband. So, assuming your family didn't kick you out for being a slut, it was the father who would have to continue to support his daughter and her child.
Adultery, it could be argued, has traditionally been seen as immoral because it is dishonest. However in some societies it is (or has been) perfectly acceptable for men to have sex with women other than their wives. In this case adultery is seen as immoral for women because they could theoretically have a child from another man that their husband would end up supporting.
Clearly if certain types of sex were immoral in and of themselves then the same standards would apply to men and women. However, even today the standards are not equal. Women who have sex with many partners are labeled whores while men who have sex with many partners are usually well liked. Our present day attitudes toward sex are derived and still greatly influenced by the rule of ancient society that a woman shouldn’t do anything to inconvenience a man. Why in America, where we pride ourselves with being so modern, are we still letting outdated sexist ideas rule our conduct?