The main reason it's a crime to marry multiple people is because of fraud and the lies involved. Some people, usually men, can sometimes run completely separate lives without either of their spouses being aware of it. Those laws were put in place to protect people from this kind of deception, and their existence in countries where women have more civil rights is no coincidence.
Marriage, as an institution, might be on the way out, and I don't have a problem with open marriages, free love, or whatever. Heff can do whatever he wants behind closed doors, but women should still be able to get back at men who lead double lives. For that reason alone, I think that Polygamy should remain as it is today. Honestly, I think it's a state law issue, so there's probably somewhere in America where it is legal, or at least not specifically illegal. Maybe you should take your harem there and everybody can live happily everafter!