How did adultery work back in the days of Solomon?  David and Solomon both had many wives and concubines.  Of course, this didn't prevent David of committing adultery with Bathsheba and eventually arranging for the murder of Uriah...but I would think having many wives and concubines at your disposal would certainly reduce the risk of committing adultery, unless I am totally misunderstanding how things like that worked back in the day.  Some Mormon's continue to believe in having polygamy, but it is wrong under new testament law.  So where is the dividing line and reason?

Just saw this today, July 23, 2018! Great question, Aaron. Adultery worked the same way it works today. It is the violation of the marital vow to be faithful to the one to whom we are married. It appears that polygamy was overlooked in the Old Testament, even as slavery was overlooked in the New. These were not God's ideal but reflected the mores of the times.

Regarding marriage, Jesus clearly taught that God's design was for one man to be married to one woman (Matthew 19:4-6). This was not so clear to the Old Testament saints, who embraced their culturally-accepted practice of polygamy,  even other practices like having children for one's wife through the wife's servant (e.g. Abraham and Hagar in view of Sarah's barrenness). Jesus had to clear this up even for the religious leaders of his day (cf. Matthew 19). 

It is not clear why God allowed things like polygamy and even slavery to continue among His people as long as He did without direct confrontation. Perhaps it was a matter of greater and lesser sins, especially when things like rampant idolatry and child sacrifice were also commonplace. There is a progress in revelation and certainly the moral understanding of Old Testament theology is quite dwarfed by the revelation of Jesus Christ, who taught morality was not simply a matter of actions but also intentions, not only of eyes, hands, and feet but also and especially of the heart. 

Proverbs 4-6 continues the parental counsel of Solomon to his son. He strongly encouraged getting wisdom and understanding and just as strongly warned against sexual immorality. As powerful as the one is to build up, the other is just as powerful to tear down. Amidst these dominant themes, there is also instruction on keeping good company, the danger of making pledges on behalf of others, the foolishness of using deception for gain, the poverty that will come to the lazy, and the things that are hateful to the LORD.

I want to key in on Proverbs 4:7,

"The beginning of wisdom is: Acquire wisdom;
And with all your acquiring, get understanding (NASB)

The KJV and NIV (alternate reading) translate the first part a little differently, "Wisdom is the principal thing" and "Wisdom is supreme" respectively. This is very instructive because we often think of the word "beginning" with respect to time, whether as chronological time or as logical time. It is the same word used in Proverbs 1:7, where we read, "The fear of the LORD is the beginning of knowledge" and Proverbs 9:10, "The fear of the LORD is the beginning of wisdom." When I researched how various translators came to one conclusion or the other, I discovered that the key factor was not grammar but context for determining meaning. However, I do think it is notable that the Hebrew word translated "beginning" or "principle thing" or "supreme" is usually translated as a temporal idea. Now, it may carry a dual sense, where often the first thing is also the most important thing. So, when we read in Matthew 6:33, "Seek first the kingdom of God and His righteousness," seeking God should not just be the first thing in time but also in priority and importance.

I see the dual sense in Proverbs 4:7, with the context (cf. Prov 4:6, "Do not forsake her...love her"; Prov 4:8, "Prize her...embrace her") strongly suggesting that wisdom is the principal thing (as expressed in the KJV). To understand the word just in its temporal sense ("beginning") would merely be stating the obvious. Of course the starting point of wisdom is to go get it. Isn't that the starting point of most everything? Nevertheless, there is this emphasis as well (cf. Prov. 4:5, "Aquire wisdom! Aquire understanding!"). So we should understand this verse both in a descriptive sense (Here's what wisdom is like: "Wisdom is supreme") and in a prescriptive sense (Here's where you start: "Aquire wisdom!")

The second part of this verse is also translated differently. For instance, the NIV translates Proverbs 4:7b as "Though it cost all you have, get understanding" while the NASB translates it as "And with all your aquiring, get understanding." You should notice that translations like the NIV tend to be more interpretive than translations like the NASB. This does not make one version better than the other. It is just something to keep in mind as you seek to discern the meaning of any text. The NIV brings out the aspect of the great worth of getting understanding. Literally, the text reads "In all your getting get," so the NIV understands this as the one task of getting of understanding should constitute not just this one but your every getting of anything, a possible Hebrew expression meaning "get it at all costs". 

The NASB translation gives a slightly different slant, seeing understanding as something separate though connected to wisdom, such that in the pursuit of wisdom, understanding should be drawn out of it. 

While texts like Proverbs 24:3 treat wisdom and understanding somewhat separately, I lean toward the NIV understanding of the text here. Solomon could have simply written, "In your getting, get," but he writes "in all your getting, get." This supports the NIV interpretation, which highlights the great worth of wisdom over the getting of anything. So NIV translates it as, "Though it cost all you have, get wisdom."

So much of living right is understanding the will of God. So we read in Galatians 5:15-17,

Be very careful, then, how you live—not as unwise but as wise, making the most of every opportunity, because the days are evil. Therefore do not be foolish, but understand what the Lord’s will is.

Wisdom for Christians is knowing what pleases the Lord and doing that and not what we think, imagine or hear from others. To gain wisdom, then, we must study God's Word. From God's Word we understand what we need to prioritize. We can know what is eternally profitable and what is passing away. We can discern what is true and significant, so that we don't waste our time, resources or energy on vain things. Wisdom is, therefore, living in such a way that we faithfully accomplish all that God has created, called and gifted us to do.