For real though, no where in the Old or New Testement is polygamy actually called a sin - it's only forbidden to deacons, elders and bishops - and no where was any polygamist actually condemend for it.
At worst it was because Solomon married pagan or foreign women who turned his attention to other gods, which had nothing to do with numbers.
Most folks claim that David was adomished by God for taking more then one wife but they don't pay attention to what God was calling him out on (taking another man's wife and killing the man to cover up the affair) but they also don't pay attention to what God was saying ("I have given you your wives. I'd give you more if you asked").
But I doubt you guys are interested in the theology or history lesson beyond me saying the guy is full of shit. But in short, you really can't say polygamy is a sin without adding to the Bible, which the Bible itself says is a no no. Not that is hasn't stopped fundies before.