In the past, humans were limited by their physical forms to a certain number of mates. Although some animals can have multiple partners (such as vultures), they are not exactly open with it and still possess an unnecessary sense of dignity about such matters. Even in ancient times, humans had to preserve their breeding stock for the future. For example, we know that Pharaohs bred slaves from different groups together so that his descendants would be able to ‘multiply’ more than if he had only married people who descended from himself.
Nowadays, humans have been liberated from their physical limitations with the introduction of new technologies such as in vitro fertilisation and genetic manipulation. Not only that, but also a sense of shame has disappeared for most people. It is now possible to mate with anyone one desires without any fear of social rejection.
However, this poses new problems. For example, with more mates available to the average person, those people are less likely to be loyal and committed towards their partners. Not only that but also they feel more entitled about their needs because there is literally anyone out there for them if things go wrong.
I do not know what the ideal solution is. I see no human society which has successfully implemented such a thing. Perhaps in future simulations, we can try to observe how different types of mating affect social cohesion and stability.