Why should monogamy remain the only legal/acceptable system in America.?
This is something of a mystery, as in the Bible we move without explanation from a society where polygamy (and concubinage) were acceptable to one where they were no longer the norm. Presumably the change was for socio-economic reasons rather than for religious ones, as it would be hard to argue for monogamy from a solely Biblical perspective. Looking at the situation objectively, if polygamy and polyandry were allowed, there would be widespread use of this to bring large numbers of people into the country as legitimate immigrants. There would be socio-economic problems as the result of many people remaining unhappily single because there were not enough partners to go round. Things would become unmanageable if both partners of a union decided to have multiple partners and children would be unsettled and insecure. There is no society in the world where there is both polygamy and polyandry, so there is no evidence of the social consequences of having both systems in operation, although
