For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Gender ratios could have had something to do with it. Without knowing more about the context it's hard to say.
For example: In the period of the early church, women converts outnumbered men.
There was a very lopsided gender ratio in the church - even greater than in society at large.
Callistus, Bishop of Rome (c. 217 - 222) urged Christian women to marry lower caste Christian men or even their own Christian slaves, something which would endanger their own social status in Rome, or "clarissima" (the loss of which would weaken the Christian community).
As a result, he sanctioned (male) concubinage - in which a high-status Christian woman could live with a lower ranking Christian man without legal marriage. This prevented mixed marriages with pagans, and the decline of Christian women's social standing in Rome.
Further reading: http://newlife.id.au/equality-and-gender-issues/tertullian-equality-in-marriage/
"Why did God allow men to have concubines in the Bible"?
In the Bible? Thomas Jefferson was the third president of the United States, he wasn't "in the Bible." Why did God allow him to have a "concubine"?
You can ask a lot of people about his wife and many won't even recall her name. Many of those same people will know the name of his concubine, Sally Hemings. She and Jefferson are said to have had six children together.The strange part is, she was Jefferson's sister-in- law; She and Jefferson's wife, Martha Wayles Jefferson had the same father, a man by the name of John Wayles. In a more civil society, Martha and Sally would have been called sisters. Like it or not, that's what they were.
"In the Bible"? Actually, not much has changed about people since "in the Bible." Laws change, and people adjust to the laws and customs of the day, but that's about the extent of it.
Joseph had some despicable brothers. Levi and Simeon slaughtered the men of Shechem because one man slept with their sister, Dinah. Judah slept with his daughter-in-law and got her pregnant. Besides all that, all ten of them sold Joseph to their distant cousins, the Ishmaelites (Gen 37:27) They were rotten to the core.
Joseph was different. Why? Abraham and Isaac lied and said their wife was their sister. Jacob was a con man. Joseph was different. There's no way he would have had a concubine, whether God ALLOWED it or not.
He kept his mind on God and God kept him in perfect peace (Isaiah 26:3).