I'm a member of a society that considers exclusivity to be a value, even among people who are not particular about who they sleep with. The one-and-one thing is rooted in the ancient human institution of jealousy, and it's not going to be rationalized away. The only time multiple partner arrangements have been existant, there was a massive power imbalance between at least one of the parties, and the others. Even Biblical kings had a wife, and concubines, the harem of Islam requires a religious belief that a woman is officially one-fourth of the human being a man is.
I just can't see a society of equal people ever adopting multiple partner relationships for more than just a fling, which most would surely regret.
If strong minority want it, it will happen in the same mode.