When I read Psalm 106:40-46, I remember the story of the Prodigal son, yet here we see an image of a people, a nation, that act out their self-indulgence, rather than one person. 
Again, let’s remember that the people were God’s people--a people from a promise made to Abraham, a people that represent Jehovah.  Just as the father of the prodigal son let his young son go, so too does Jehovah give way to his people, to their free will, to do as they wish.  In his anger toward his people, God let them go and allow their enemy to treat them as they wish. Just as Egypt enslaved them, so too do other nations subject them to their power. 
The story of God’s people can be a reflection of how we live today--in self-indulgence, not all of us, but many of us.  We live as if there is no relationship between God and us.  
However, in God’s great love, he continued to deliver his people.  Today, he still gives us deliverance through Christ, our Lord.  Jesus was born, lived, died, and rose again to deliver us from sin (self-indulgence) and death.  Jesus chooses mercy as a Father who loves his child would give mercy.

     Losing your identity is very harmful to yourself, much more to a people group, i.e., the Hebrew people. The Psalmist of 106 verses 34-39 continues his historical reflection of his people's journey through the wilderness and into Canaan. In these particular verses, the Hebrew people find themselves in the promised land.  The very last verse gives us a visionary tale of their story after arriving in the promised land, "they prostituted themselves" to a people that did not know God, Jehovah. 
     What can we learn from their story, from the slavery of another people to slavery of self-indulgence, neglecting God?  To answer this question, we must consider our spiritual identity.  Who are we in relationship to God, creator of the heavens, heavenly father to Christ? Once we can identify with Christ, as Lord, then we can move forward to answer the question about learning from the passage in psalms. 
     The lesson is to be careful whom we hang with, not culturally, but in terms of morality.  Yes, we can befriend people with different cultures, unlike the Hebrews in this passage.  Their life was about establishing a people who are monotheistic in their belief system.  They moved into a polytheistic neighborhood, that believed in many gods, and sacrificed their children to their gods.  Since Christ arrived and sacrificed himself for humanity, we do not need to do what the Hebrew people were to do in their history, as the Bible reveals.  
     We can befriend others, hang with folks that are not like us, and be careful with boundaries.  We can hang with others, that struggle with morals, show them the love of Christ, be the light of the world.  Again, know yourself and know your boundaries and keep your limits. We can do all this through the Bible; the psalmist elsewhere stated that he hides the Word in his heart, that he may not sin against God. The Bible informs our morals as Christians.