How do you explain God giving David his wives through his prophet Nathan?
2 Sam. 12: 8
8 And I gave thee thy master’s house, and thy master’s wives into thy bosom,
In part, it's because (a) you don't know Hebrew; and (b) you've isolated 2 Sam 12 from 2 Sam 16 (& possibly 2 Sam 6). But certainly even if we agree on what to call these women, 2 Sam 12:8 is still troublesome.
Allow me to explain: First of all the Hebrew word used of these women in 2 Sam 12 is "issah"--which can mean either woman or wife and is even used in one verse in Judges for "concubine." Secondly, Nathan prophesies, among other things, that the Lord is going to take these women and give them to one who is close to you--lying with them in broad daylight. (2 Sam. 12:11)
Now when did this prophesy come to fruition? 2 Sam. 16:21-22 says that Ahithophel tells David's son, Absalom, to "Lie with your father's concubines...So they pitched a tent for Absalom on the roof, and he lay with his father's concubines in the sight of all Israel."
The word here for "concubines" is pileges.
So, my main point? Simply put, in context, these women are "concubines." Maybe even the "slave girls" referenced in 2 Sam. 6:22.
Biblical support:
(1) 2 Sam. 16:21-22 identifies these women as "concubines."
(2) The Hebrew word used in 2 Sam. 12 to describe these same women is Issah--which can mean "woman" & is used in Judges 19:26 to mean "concubine." In fact, Issah is even used in Gen 7:2 to describe the female mate of animals. [So while it is at times used in an endearing way (like "bride" in Gen 29:21 and Deut. 20:7), it isn't consisently the case.]
(3) When David's wife, Michal was upset with David for disrobing in the sight of slave girls (2 Sam. 6:20), the implication in 2 Sam. 6:23 is that they may not have slept together from that point onward...And David says then, "I will become even more undignified than this, and I will be humiliated in my own eyes. But by these slave girls you spoke of, I will be held in honor." (2 Sam. 6:22) It's possible here that David is projecting that he will choose to sleep with slave girls rather than his wife...these slave girls will "honor" him--something his wife isn't doing (she accused him of public vulgarity)--but in doing so, he is "humiliating himself in his own eyes" in engaging in an "undignified" manner.
That Still Doesn't Clear Everything Up
Now, suppose we agree these women are David's concubines. In what way does that elevate my point, but in what way is it still troublesome?
Well, it elevates my point in that I contend that God created only one woman for Adam--not a harem. And that God has seen marriage as an institution of one man and one woman. (And I'll note that your church general authorities now agree with me on that point).
If these women are indeed "concubines" as 2 Sam. 16:21-22 says (and the interpretation is open for that in 2 Sam. 12 as well)...then marriage as a one-woman enterprise is still protected.
Still, where is the "troublesome" dimension of this? The verse you pointed out, 2 Sam. 12:8, clearly says that Nathan is speaking for God, & that God gave these women into his arms. Therefore, God was assuming responsibility for placing concubines in David's household! (That's the part I can't fully explain)
Why did he do this?
I would first surmise it's for the same reason he "allowed" Caleb & Abram & Jacob to have concubines without calling down his judgment. At least in those other cases, there is nothing indicative that God condoned or directed it.
Beyond that, it's speculation. For example, I could speculate that these concubines were indeed the "slave girls" referenced in 2 Sam. 6. If that's the case, the Bible hasn't left us clueless that even when David took them as concubines, he himself knew that what he was doing was "undignified"--bringing shame & humiliation even in his own eyes (2 Sam. 6:22). (The message then for the rest of us is not to engage in humiliating, undignified sexual behavior)
I could also futher speculate that God allowed David to sleep with the concubines because he knew of the rift between David and Michal--one that would extend to their procreative life (2 Sam. 6:23). God knew that the eventual sexual pathway David chose would lead to two deaths--that of Uriah the Hittite and that of his own infant son. Was God opening a "fork in the sexual road" that would give him an option of the lesser of two evils? I draw this speculation from Nathan: "And if all this had been too little, I would have given you even more. Why did you despise the word of the Lord by doing what is evil in his eyes?" (2 Sam. 12:8-9) (The lesson for the rest of us might be that if we engage in adultery that leads to say, an abortion, that is both a heavy price for the child to pay for our sin...plus to murder someone to take their wife despises the Word of the Lord.)
But all this speculation only opens up new problematic doors. For example, the fact that Bathsheba is Jesus' grandmother several times over. (Why would God try to divert David's sexual energy from someone who would be an ancestor of Jesus?)
In the final view of things, what does seem to make some sense to me is that even when adultery is involved (Bathsheba); even when death results due to adultery (Uriah & David's child); even when concubines/slave girls/mistresses are used for sexual purposes; even when marital relations run dry (Michal) when the husband is no longer honored; God is not surprised by our sin. He works in the very midst of our sin. He doesn't cover it up (David is humiliated & Nathan shines a light on his sin); and yet He eventually does cover it up thru the blood of the Lamb ("The Lord has taken away your sin"--2 Sam. 12:13...but not all the consequences of your sin).
The key thing in all of this is not to "glorify" either marital polygamy or the sexual utilization of concubines. I stand by my contention that no verse in Scripture shows God as directing or condoning marital polygamy--but that He indeed "allowed" it to occur. The institution of marriage in God's eyes has been & remains sacred. (The fact that I can't fully explain God's view of concubines beyond marriage in every case is just another reminder that I don't it all...that we "see thru a glass darkly").