I read an interesting take on *why* some people are sluts, for purely biological reasons. It is based on the idea that many people will not produce and raise optimal offspring, so sluts act to inhibit their reproduction, like sterile fruit flies.
Acting in a complementary manner to all the other biological and social reproductive processes, that work to produce the best offspring being raised by the best parents, slutty behavior is designed to 'distract' those who are not optimal breeders.
Assume you have 5 males and 5 females. Only three of each are the best breeding pairs. This leaves two couples that need to "get out of the way", as otherwise they might interfere with the breeding pairs.
Infertility isn't enough, as that could still interfere; so what is needed is someone who is willing to engage both of the other sex that shouldn't have offspring.
But it doesn't stop there. Biology dictates a window of time for optimal reproduction. Therefore, a post-menopausal woman is ideal to distract younger, undesirable males away from the better females. An older man, likewise, can distract younger, undesirable females.
This also is a possible explanation for some types of homosexuality. Biology dictates that they shouldn't reproduce.
All told, legitimate biological reasons may explain much of traditional human "bad" behavior.
Interesting theory. So, that would mean if we forced them to act heterosexually, and marry, then those genes would be passed on to future generations? That's another good reason to let them have each other, and keep straight people from accidentally marrying a homosexual. Of course, this all goes out the window with lesbians with turkey basters...