This is a quick summary I once gave someone on the subject:

The condemnation and rejection of multiple wives came from ancient, pagan Rome, and was spread and enforced by the Roman Catholics; then the Mormon cult gave multiple wives a bad name, so that America outlawed it, and most of the world copied America in that evil.
Abraham, the friend of God, who kept God's law, married multiple wives. As friends of God, who keep his law, we must stand against the pagan beliefs that condemn those who, like our father Abraham, marry multiple wives.