Law should not be based upon rather or not something is sinful. We live in a democratic republic, not a theocracy. Also, outlawing it in other countries, especially where Christianity is not the majority religion, is laughable, the amazing expenditure of lives that this would cost.
Polygamy is sinful. There are only man made, lustful reasons for polygamy.
I would like to see it outlawed everywhere. I was told at an early age I would share a husband in heaven with other mormon women. It was traumatic and I remember crying several times and saying I did not want to go to heaven.
My heart goes out to every little girl down through the ages who has been given away to some dirty old man.