Poor shamus...looking for an argument anywhere :)
Yeru, I would say that "societal argument" is a null term. Your beliefs fuel your opinion on what's best for society.
Polygamy has been rejected by this society on religious grounds - but there are no biological imperatives working against the adoption of polygamy in the future...but there are economic and demographic reasons that polygamy is unlikely to be on an agenda anytime soon. But it was in the bible... it could only "work" in a society or subculture where women are not valued highly. The questions Yeru raises about divorce can be easily addressed in any contract - plural marriage could make sense in the future, but there seems to be no impetus for it in our culture today except from foreigners and Mormon-secessionist cults who soak the US for welfare (see economic argument above). Polygamy existed as a survival tool for genes back when survival was the key question for all societies. Now that success is the key question, our thousands of years of conditioning and acculturation aren't applicable as often.
Incest is obviously a biological event that should be avoided for biological reasons. Pedophilia is abuse of a minor and needs no discussion. (Those who compare homosexuality and pedophilia seem to be unclear on when someone involved is being harmed and when no one is being harmed.)