The Ten Commandments are an expression of God's Holy nature ... He is not a murderer so 'thou shaltnot murder'; He is not an adulterer so 'thou shalt not commit adultery', and so on. The Israelites demand that Moses go ask God what they could do (they wanted to work their way to exaltation) to be righteous before God. Of course, lacking God's nature within them, they could not possibly live as equal to His exalted nature, reflected perfectly with the Commandments the people could not live up to 100%. That is the reason Christ took a tabernacle of flesh, dwelt among us as God with us, and gave His life/blood a ransom for our unrighteousness. Until you comprhend just how far from able to become exalted via even some tremendous works we humans are, you will not comprehend the unspeakable mercy and Grace of God in Christ Jesus.
That you would mistakenly believe God would go against His nature (He's not an adulterer so 'thou shalt not commit adultery') and give Saul's wives to David while Saul was yet alive tells us so much about the agenda of apologists willing to even contradict God's Holy Nature in order to make a place for smithism/Mormonism to exist in contradiction to God's nature.
Tantiboh already answered the difference between ploygamy and plural wives. There is a difference between being married for time and etenrity. Some plural marraiges are conjugal and some are not. Some of Brigham Young's wives lived as wives with whom he had children and others were old widows he cared for but had no conjugal realtions with.
My grandfather's wife was sealed to another man for time and all eternity in the temple. He died. So she is no longer married for time (which has ended for them as a couple) but is still sealed for eternity to him. She has now married my grandfather in the temple for time but was not sealed for eternity. She will still be sealed to her first husband (not my grandfather) after her death.
Joseph did not have conjugal relations with other men's wives even though he may have been "sealed" to them for the hereafter. Insinuations aside there is no proof he ever committed adultery with other men's wives. At the time even some of chief detractors had to admit they had no proof of him committing adultery.
From wiki there is a basic explanation of the difference between a sealing and a marriage.
[edit] Sealing vs Marriage - There is a subtle difference between 'sealing' (which is a priesthood ordinance that binds individuals together in the eternities), and 'marriage' (a social tradition in which the man and woman agree to be husband and wife in this life). In those early days of this religion, common practices and doctrines were not yet well-defined. It is speculated that many, if not all, of the arrangements between Smith and these women were more in alignment with the concept of sealings, not marriages, in the sense that Smith did not join with any of these women, except for Emma, in a family unit.
Even among those who accept the views of conventional historians, there is disagreement as to the precise number of wives Smith had: the LDS Church's genealogical website lists 24 marriages for Smith, four of which are indicated to have taken place after his death.[11] Fawn M. Brodie lists 48 (Brodie 1971, p. 457), D. Michael Quinn 46 (Quinn 1994, p. 587), and George D. Smith 42 (Smith 1994, p. 14). The discrepancy is created by the lack of documents to support the alleged marriages to some of the named wives.
A number of Smith's "marriages" occurred after his death, with the wife being sealed to Joseph via a proxy that stood in for him.[12] One historian, Todd M. Compton, documented at least thirty-three plural marriages or sealings during Smith's lifetime (Compton Dec. 1996). Many people think it is without question that Joseph had multiple wives; but, as Compton states multiple times in his work, "Absolutely nothing is known of this marriage after the ceremony"—that is, it is unclear how many of the women Smith had sexual relations with. There are allegations that Smith had at least one child born to a plural wife, but this remains unproven (Compton Summer 1996, p. 17,29).
David's wife Michal was given to another man (though not technically divorced from David) he took her back but did not have sexual realtions with her, yet she was considered his wife still. Plural marriage does not always include conjugality.
Adultery or sleeping with another man's wife even if sealed to them was expressly forbidden and would warrant excommunication. - from wiki The first mayor of Nauvoo, John C. Bennett, a recent convert to the church, was excommunicated for spiritual wifery, a very different practice than plural marriage. He had used the emerging teachings on eternal and plural marriage to justify seduction, adultery and, in some cases, the practice of abortion.