I don't want polygamy in the US, but as far as it being a sin. I'm not so sure that is a sin or that it ever was a sin. In Israel if a woman becomes a widow before having children, it was a responsibility of one of her husband's brothers to marry her for the sake of her having a child.
That's of a special case of a situation, but it when reading the bible it seems like the people having multiple wives were never a sin that was renounced or forbidden. That and when people did have multiple wife's that wasn't one of the sins that was described by the prophets when they warned Israel or other nations of their downfall due to the sins they commited.
I don't know, I think there are lessons from Jacob's life on having two wives, and later there was a law or a proverb to not marry two sisters because they would be in competation with eachother, (as well as a law to not marry a mother and a daughter, so the laws do get specific in that area). However aside from the lessons of trying to hold down a family with competing siblinings from different mothers, or wives competing with eachother, I don't think the bible forbids poligamy as a sin.
I don't like it though. I think polygamy destabilizes the family and destabilizes a marriage. It's better then an open relationship, or an open marriage, but only just barely in my opinion as far as being the reason relationships fall apart.