Well I think I understand why a community is more believable, but Genesis doesn't say that Adam and Eve were an entire community. It says that Eve was the mother of every person ever born. If Eve wasn't the mother of all the living then the story it could be that Adam was all the men and Eve was all the women; and they might have all been one big orgy. I don't disagree with you completely. I think that the idea of multiple wives per man was from early times considered to be a necessary evil, and I think that eventually it was outlawed when it was no longer seen to be necessary. Even in modern times you have the occasional polygamist community which tries to argue that polygamy solves certain problems. It can solve certain problems but also causes certain evils and its just not necessary anymore. Hence it is outlawed.