Should polygamy remain illegal?

So I'm not into this myself, but I've always wondered why polygamy is illegal. It just doesn't make sense to me. If someone is happier with two husbands or two wives or whatever then what is really the harm? In Canada you can technically get 5 years in jail for it, and I googled what the penalties are in the US and found out that you can get jail time for it in many states as well. It seems like a law that's kind of there for no reason... but maybe I'm missing some important factor that should make it remain illegal? What does everyone else think?

Also according to Google polygamy is legal or legal for certain populations of people in India, Malaysia, the Philippines, Singapore, Indonesia, Pakistan, Nigeria, Kenya, Afghanistan, and a bunch of other places as well.