Question: "What is a concubine? Why did God allow men to have concubines in the Bible?"
Answer: A concubine is a female who voluntarily enslaves and sells herself to a man primarily for his sexual pleasure. Concubines in the patriarchal age and beyond did not have equal status with a wife. A concubine could not marry her master because of her slave status, although, for her, the relationship was exclusive and ongoing. Sometimes concubines were used to bear children for men whose wives were barren. Concubines in Israel possessed many of the same rights as legitimate wives, without the same respect.
Although itís true the Bible nowhere explicitly condemns concubinage, a condemnation can be found implicitly from the beginning of time. According to Genesis 2:21-24, Godís original intent was for marriage to be between one man and one woman, and that has never changed (Genesis 1:27). As a matter of fact, a study of the lives of men like King David and King Solomon (who had 300 concubines; 1 Kings 11:3) reveals that many of their problems stemmed from polygamous relationships (2 Samuel 11:2-4).
The Bible never explains why God allowed men to have concubines. He allowed divorce and polygamy, too, although neither was part of His original plan for marriage. Jesus said God allowed divorce because of the hardness of menís hearts (Matthew 19:8). We can assume the same hardness of heart led to polygamy and concubinage.
We can also surmise a reason based on the culture of the day. Unmarried women in ancient times were completely dependent on their family members, such as their fathers, brothers, etc. If for some reason a woman had no family members or her husband had died or divorced her, she would be left with few options for survival. Most women in ancient times were uneducated and unskilled in a trade. Providing for themselves was very difficult, and they were vulnerable to those who would prey upon them. For many women in dire situations, becoming a concubine was a much more suitable option than prostitution, homelessness, or death. At least a concubine would be provided a home and afforded a certain amount of care.
It appears God allowed the sin of concubinage, in part, to provide for women in need, although it was certainly not an ideal situation. Sin is never ideal. Christians should be reminded that, just because God allows a sin for a time, it does not mean God is pleased with it. Many Bible narratives teach that God can take what some people mean for evil and use it for good (e.g., Genesis 50:20).