The fullness of the priesthood is not given to us when we are sealed in the temple. We learn in D&C 132 that King David received all his wives through eternal marriage.
D&C 132:39 David’s wives and concubines were given unto him of me, by the hand of Nathan, my servant, and others of the prophets who had the keys of this power
But Joseph Smith also explained: Although David was a king, he never did obtain the spirit and power of Elijah and the fullness of the Priesthood; and the Priesthood that he received, and the throne and kingdom of David is to be taken from him and given to another by the name of David in the last days, raised up out of his lineage.
Thus entering eternal marriage was not sufficient for David to gain the power of Elijah and the fullness of the priesthood.