So, over the years monkeys got mutated and became human beings. And one mutation led to the loss of promiscuity in the major chunk of human beings. Let us go back to ancient times. Whatever proof we have says that in some regions of earth polygamy was prevalent, in some polyandry and in others monogamy. This is about marriage only, to simply things, I assume that copulation only took place after marriage or if before marriage then it resulted in marriage, but they were inseparable. So, humans were always confused what to do? They pondered but then they had multiple partners. In some societies powerful men were polygamous, but not all men. So, there was no clear cut rule defining this human behavior.
There was a formation of a societal institution called marriage, because of which free sex was hindered. There was no free sex in the world, man and woman entered a relationship before having sex, which was recognized by the society. This was a part of civilization. But still they were not sure about whether a man should have multiple partners or a woman. This thing continued for long, and then with multiple factors like awareness, social revolutions things changed. Now, we have a society where we believe in one marriage!
A lot of people think that copulation cannot be had without feelings. That is true. But feeling of attachment is not a must. You can copulate without affection. It is a physical need related to psyche. But that psyche need not be affection.
Essentially we still are unable to prove what is wrong with copulating with multiple partners. Apart from lame reasons that we give about AIDS and all, well these can be good reasons but still not strong enough. I believe has to be psychological bent towards this need analysis. What exactly is it determined by?
(In complete! First of the letters exchanged with a lovely contemplator)