In the new Essay called Plural Marriage in Kirtland and Nauvoo under the header "Plural Marriage and Eternal Marriage" it explains that polygamous sealings are eternal marriages as well. It says:
"As early as 1840, Joseph Smith privately taught Apostle Parley P. Pratt that the 'heavenly order' allowed Pratt and his wife to be together 'for time and all eternity.' Joseph also taught that men like Pratt—who had remarried following the death of his first wife—could be married (or sealed) to their wives for eternity, under the proper conditions."
----------------------------------------­--------------------
Quotes about Eternal Marriage:

"The doctrine of the family begins with heavenly parents. Our highest aspiration is to be like them."

"Families can be together forever. To enjoy this blessing we must be married in the temple. When people are married outside the temple, the marriage ends when one of the partners dies. When we are married in the temple by the authority of the Melchizedek Priesthood, we are married for time and eternity. If we keep our covenants with the Lord, our families will be united eternally as husband, wife, and children. Death cannot separate us."

"The Doctrine and Covenants sets forth the eternal nature of the marriage relationship and the family. Celestial marriage and a continuation of the family unit enable husbands and wives to become gods (D&C 132: 15-20)."

"Marriage performed under the law of the gospel and the holy priesthood is for mortal life and for eternity. Worthy men and women thus sealed in the temple in marriage may continue as husband and wife throughout eternity."