<p>^ I'm not a girl so I have no idea if this is anywhere close to true but I think that if a guy mentions casually that he has had some relationships another girl may take that as a bona fide.</p>
The Parental Investment Theory suggests that the sex making the greatest investment in raising the young (for humans the female) will be more discriminating in mating than the sex that invests less for the offspring. (the male) This is because (successful) reproduction will cost the female much greater resources than it would for the male. However, the theory also states that the sex investing less resources will also be more reproductively competitive amongst each other. </p>
<p>Therefore. When a guy sleeps with a lot of girls, he is congratulated by his friends because he has successfully demonstrated his ability to win the sexual selection game. When a girl sleeps with a lot of guys, she is called a slut because she has carelessly been receptive to numerous um... genetic material without considering the costs of reproduction to herself or her potential offspring.
<p>Or to put it simply: "If a key opens a bunch of different locks then it is awesome because it's a master key. If a lock is opened by a bunch of different keys then it is a really crappy lock.</p>