JOEL - The Doctrine and Covenants Section 132 explaining the doctrine of plural marraige has a date of July 1843. Actually Joseph Smith received revelation about this doctrine ten years before this.
Here is what he said a few months after the writing of D&C 132, which is recorded in both the History of the Church and the Teachings of the Prophet Joseph Smith references:

"In the afternoon, rode to the prairie to show some of the brethren some land. Evening, at home, and walked up and down the streets with my scribe. Gave instructions to try those persons who were preaching, teaching, or practicing the doctrine of plurality of wives: for, according to the law, I hold the keys of this power in the last days; for there is never but one on earth at a time on whom the power and its keys are conferred; and I have constantly said no man shall have but one wife at a time, unless the Lord directs otherwise."

The revelation on plural marriage was written before Joseph made this statement, but the Prophet is not preaching against the doctrine here. He is addresing an issue in his statement concerning those claiming to have authority to perform plural marriages within the Church. That authority is given only to one person on the earth at a time. He is also explaining that no man should practice it "unless the Lord directs otherwise". This is something that he constantly taught for the years preceding the D&C 132 revelation on it. Even though as of July the doctrine was now in place, it could not be practiced unless the Lord directed it through His prophet. The Lord was not directing those other people to preach or perform the ordinance.