Originally Posted by Master_Shake

If by polygamy you mean having sex with more than one person at a time, or while you are in a relationship with someone, then nothing is morally wrong, but watch out for STD's.

If by polygamy you mean marriage to more than one person at the same time, then the problem is the same thing that's wrong with marriage to one person. Marriage is a institution dedicated to ownership of other people, and controlling interests in the property of others. It's consensual slavery.