Q. What is the difference between wives and concubines in the Bible? I understand wives had higher status and that Abraham’s and Jacob’s concubines were their wives’ servants. Is concubine basically a technical term for servants that double as sex slaves? Or did they actually have rights within the family structure?
There is no question that concubinage was an exploitative practice. However, women who were concubines were not exploited primarily for sex. They were exploited for the children they could have. In the agricultural Old Testament culture, children were needed to work the land, and they were also needed to carry on the family name and preserve family rights to property. So most typically, men would marry concubines when their wives could not have children or when men felt they needed more children.
A concubine was legally married to the man whose concubine she was. We see this, for example, in the terminology of “father-in-law” and “son-in-law” that is used in one Old Testament account for the relationship between a man and the father of his concubine. But a concubine had a lower status than a wife.
The difference in status was not that the wife was free while the concubine was a slave. It is true that the Old Testament discusses cases in which a man might marry one of his female slaves, who would then become his concubine as well. It is also true, as you noted, that a man could marry one of his wife’s female slaves as a concubine. So there was a connection between concubinage and another very exploitative practice, slavery.
But the essential difference between a wife and a concubine was that the children of the wife were certain to have inheritance rights to the property of their father, while the children of the concubine did not necessarily have such rights. I think it would probably be too much to say that children of concubines could not inherit from their father, but their situation was very tenuous.
For example. when Abraham’s wife Sarah could not have children, she had him marry her female slave Hagar so that she could adopt the son of Hagar. But when Sarah later had a son of her own, Isaac, she insisted that Abraham send Hagar and her son Ishmael away so that only Isaac would inherit. After Sarah died, Abraham married a woman named Keturah as a concubine, but he gave her sons gifts in lieu of inheritance and sent them away as well.
By contrast, when Jacob married Bilhah and Zilpah, the slaves of his two wives Rachel and Leah, in order to have more children, he gave the sons of Bilhah and Zilpah full inheritance rights along with the sons of Rachel and Leah.
But in general the position of concubines and their children within the family structure was very insecure. It seems that women who were already in a vulnerable position, for example, as slaves or foreigners or both, were further exploited as concubines for the children they could have. Later in Israelite history, kings would marry many concubines as a symbol of royal prestige and perhaps to pursue certain political ends. These women were not being exploited for their children, since such kings already had many wives and many children by them, but they were still being exploited for those other reasons.
So I think it would not be quite accurate to describe a concubine as a “secondary wife.” While she was legally married, her situation was so different from that of an actual wife that I think a separate term should be used to identify it. Marriage is meant to be a relationship characterized by mutuality and equality. The power differential in concubinage is so great that it is not true marriage. And so I believe we should work to eliminate the practice of concubinage in our world today, just as we should work to eliminate slavery. The fact that concubinage is depicted and described in the Bible does not indicate any sanction for it or approval on God’s part.
Good Question 
So when and how did this practice of having multiple wives and concubines change to the point where and when Herod Antipas was accused of and guilty of adultery?
John the Baptist did not accuse Herod Antipas of adultery (having sexual relations with someone else’s wife). And John’s objection was not that Herod had more than one wife. Herod divorced his wife, and Herodias divorced her husband, so that they could marry. Rather, John insisted that it was not lawful for Herod to be married, while his brother was alive, to a woman who had been married to his brother. Leviticus 18:16 forbade this. So John accused Herod of incest, not adultery or bigamy.