The fact is: prior to the Christian New Covenant, God did allow concubines and it was not adultery! Adultery was taking another man's wife (or another woman's husband),
Did Jesus see a difference in a legal marriage and a 'live-in "
"16 He told her, "Go, call your husband and come back." 17 "I have no husband," she replied. Jesus said to her, "You are right when you say you have no husband. 18 The fact is, you have had five husbands, and the man you now have is not your husband. What you have just said is quite true." (John 4 NIV)
Further evidence that marriage is a legal matter is seen in the Bible's very extensive use of mother-in-law, father-in-law, daughter-in-law and son-in-law.
The practise of a man having more than one wife or concubines continued into the Roman society of Jesus' day but although no single statement of Jesus or Paul completely barred this approach for Christians it starts to become clear that the practise is hardly consistent with the Christian life. A consideration of Jesus' comments in Matthew 5-7, Matthew 19:1-9 and perhaps especially Paul's comments on marital love in 1 Corinthians 7 tell us much more. Paul assumes either no marriage or monogamous marriage within the Christian life
So why did God ever allow such things?
I think that here we have to remind ourselves that we cannot question the choices or decisions of God!! In the very early years after Creation, for instance, men would necessarily have had to marry their close female relations, but later on this became barred with strict laws against incest. Are we going to question God over this? What right would we have to question the wisdom of our Creator? Then, for a long period of time, plural marriages, concubinage and very large families were allowed (and the latter certainly encouraged), and there is ample reason to think that the need to re-build the world's population, post-flood, may well have been a reason for this. But where these things were taken to excess, the Lord issued warnings. See Genesis 6:1-2 for instance.
"6 When human beings began to increase in number on the earth and daughters were born to them, 2 the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose.
Which case greed & lust were present with no thought of comitment or upright purpose.