That seems like a very convenient inference or interpretation of scripture by man, not the word of god himself. Especially since the bible tells you what to do if you do take a second (or third, or 700th) wife. "If he takes another wife to himself, he shall not diminish her food, her clothing, or her marital rights." (Exodus 21:10). Why would it bother telling you that instead of saying not to do it at all, if it is, in fact, a sin? I don't see God doing the same thing with, say, the Golden Rule. "Love thy neighbor, but if you really can't, at least act like you do to mitigate the sin somewhat."
"Thus saith the LORD God of Israel, I anointed thee king over Israel, and I delivered thee out of the hand of Saul; And I gave thee thy master's house, and thy master's wives into thy bosom, and gave thee the house of Israel and of Judah; and if that had been too little, I would moreover have given unto thee such and such things." (2 Samuel 12:7-8). Sure sounds like God giving David many wives to me. Why would God do that if it was a sin?