This was the reality in ancient times. Indeed, we don't want to see this happen today, and looking back at the history, we could be condemnatory, or we can just appreciate the social milieu, the context that was being practiced. We can try to understand why this was being practiced. Why was it allowed? Need to think about what we're going to do about it today. These are separate questions.
In 1949, China had concubinage that was finally ruled out. Usually, concubinage was linked to slavery. A woman was captured as a slave or sold in the open market. Perhaps she was captured in war, she and her people were enslaved. Once she was a slave, she might've been drawn into her master's harem as one of his sexual partners. She wouldn't have the status of a wife, but she would be the status of a concubine. Until recently, if slavery persisted in the world, people had concubines. Specifically, men in power had concubines.
They did not necessarily have to be in power. They must have enough wherewithal to acquire a concubine. For example, if he was a soldier in war, we read in the Old Testament, that the soldiers went out, and killed so many men. Usually, they captured young women. When the young women were captured, they were distributed among the soldiers. If he was a soldier in battle, then he didn't necessarily have to be very influential in society. He might have received a slave woman that would serve as his concubine. However, if slaves were sold in the open market, then he had the wealth to buy one, and he could acquire a concubine too. Yet, concubinage is not necessarily tied to slavery. We see widespread practice in the ancient world, across cultures. This was almost universal where women might become concubines. A person in power may deliberately choose one for himself. A Monarch has a sexual interest in a particular woman.
She may not be able to resist the advances of the Monarch, but there may be other circumstances where it is more advantageous for a woman, to be a concubine to a man of great influence, wealth, and power than to be married to an average person in the society.
According to the historical discussions, one advantage is that if she bore a boy for her influential master, the child himself, her son would be very influential in that society as well, just by virtue of being a child of the master.
She may choose that as a long-term strategy for herself. It's a bad situation that women to make such choices. In modern times, a woman has the full liberty to decide what would be advantageous to her without being an exploitative sort of relationship. Indeed, power imbalance still exists in modern times.
However, in the past, it wasn't in people's minds like consent. The whole idea of being an imbalance in power, and one person taking advantage of the other, and the whole idea of agency. A person has agency to be able to choose their path. Choose who they want to marry. All those things were very foreign. They came about quite recently.
On the other hand, there were other factors that led to polygamy. On the other hand, concubinage is an extension of the polygamy issue. One of the factors was that if a tribe remained small, then they were often the targets of attack by stronger tribes. A smaller pack of wolves might be attacked by a larger pack of wolves. However, a smaller tribe may be attacked by a larger hoard, of marauding invaders. The way of survival in the past was for families to grow into clans that grew into larger tribes. The larger the tribe, the more secure they were.
In terms of manpower, they had young fighting warriors to defend the tribe. A larger tribe grew by having everybody related to each other. Specifically, this was accomplished through a patriarch. Theoretically, a man could be at the head of a massive family. On the other hand, the woman had a more limited scope biologically. A man could father so many children by having multiple wives and concubines. Whereas, a woman would be limited to how many children she can bear, realistically, in her lifetime. This is how you grew a large family. If a man wanted to grow a large family, he would marry multiple women. Yet, he had legal obligations to each wife. However, if he had concubines, in addition to his legally wedded wives, then he could father more children through the concubines, and he wouldn't have so many legal obligations.
What sort of obligations were imposed upon men if they had concubines?
What is mentioned in historical discussions is inheritance. According to the Bible, Sarah had a handmaid, Hagar, whom she gave to Abraham to be his wife. This might be a euphemism that he could have intimate relations with her. Whereas, she remains in the status of a concubine, in the biblical context. Muslims regard Hagar as the wife of Abraham. However, in the Bible, she's regarded as a concubine. Sarah said to Abraham that the child of this handmaid, that is Ishmael, is not going to inherit along with my son. That is Isaac, whom Muslims know to be as Ishaq. There is an intimation that the child of the concubine will not share in the inheritance. A man is married. He has his wife or wives, and his existing wives have a certain claim on him. If he has a larger sexual interest or wants to build a larger family, he may not be able to take another wife without upsetting the existing wives. They may impose upon him; we cannot grant for you to take another wife. However, you can take a concubine. The concubine would have a lesser status. The existing wives could look down upon her. They can allow for somebody of lesser status to be included in the family, through which their husband would have more children. The inheritance of their own children is secure. These are the dynamics and the various considerations that led generally to the practice of concubinage in ancient societies.
They had different lenses to see those problems. One of the lenses that they had was the problem of the power imbalance. They needed the power imbalance. It’s like somebody needed to have a show of power. The strength of the tribe, but there was also the consideration of the power of the individual. A Monarch, a King, or a leader, had to show his power and strength. A male had to show his virility by having many wives and concubines. He showed his masculinity, social status, and power. In a modern context, the person would have so many possessions and status symbols. This was a status symbol.
According to the Bible, when one of the sons of David wanted to show his power, he slept with his father's concubines in the sight of all Israel. Meaning, he took them to be his legitimate consorts as a matter of public knowledge. Consequently, he would be establishing himself in a position of power. This was somebody to be reckoned with.