"Concubine" refers to a woman who is in a steady relationship with a man. However, she is not legally married to him. There's a distinction between the wife and the concubine, a mutually exclusive definition. The wife is not the concubine, and the concubine is not the wife. Yet, the concubine is like a wife, but she has a secondary status.
What sort of questions does this raise? Was Prophet Muhammad (PBUH) engaging in immoral behavior? What does it mean for Muslims that try to follow Prophet Muhammad (PBUH)? If the Quran mentions concubine age, or having sex with slaves, what does that mean about the Quran and this text from God?
Many concubines started out as slaves. If you imagine the situation where a woman is a slave, meaning that she might've been captured in war, she might've been even sold in the open market. This is a terrible occurrence for concubines. However, it happens in the past. If a woman is taken and used for sexual purposes, then there is a combined problem. Generally, one is the problem of slavery that applied to men and women. This specific problem for women is that not only is she's a slave, but she's used for the sexual pleasure of her master. This is so unacceptable today. How are Muslims going to view the reported actions of Prophet Muhammad (PBUH)? Did he have a concubine that he used as a wife? If so, what does that imply? Does that mean that we must follow him in everything that he did? There are some things that he did as a human being who will live a long time ago, he acted as a morally upright person, and one of the best exemplars for his time and place. Also, he is an example for all time to follow, even in all these minor details. We would be a state of confusion, knowing that that is not acceptable today. However, if circumstances would permit, one would do the same action. However, most Muslims recognized that they would not do that today.
Are many Muslims are not aware that Prophet Muhammad (PBUH) had concubines? That it wasn't even an issue.
Indeed, Muslims are becoming aware because of the internet and information is being shared and a non-Muslim may send a bit of information about concubines. There would be referring to some reports in our classical sources stating that Prophet Muhammad (PBUH) had a concubine. How does Muslims regard those reports? Does Muslims have to take those reports as being factual? Muslims might look at those reports with a certain reasonable degree of skepticism and look at the possibility that some mistake about the viewpoint of Prophet Muhammad (PBUH).
It is likely that Maria, the Coptic woman, was Prophet Muhammad’s (PBUH) wife, but people misconstrued as a genuine mistake or for their own purposes. Hence, that's one aspect of the life of the Prophet Muhammad (PBUH) that Muslims must research in greater detail.
How does Muslims regard the Quran? Is there a verse in the Quran that states a man could have a concubine? Does he have the right to sleep with her? Does the Quran promote the marriage of the female slaves? If a female slave was in a sexual relationship with a man, then it would be her legitimately married husband? There is a problem of war that reels from the blood, violence of war, and want peace. During historical realities, when war was very frequent and people were captured in war, what happened with the prisoners of war?
Consequently, the special problem of the prisoners of war was dealt with in a unique way during the Medina society of Prophet Muhammad (PBUH). They were taken and distributed as domestic servants within the Muslim community.
In the social context, this was the humane way of dealing with the slaves. Muslims were encouraged to feed them with the same quality of food and give clothing with the same quality that one would wear. Everyone did not follow all that those ideals, but within that sort of ideal thoughts. A woman was one of the slaves who were distributed and became a domestic servant. If there was some attraction between her and her master, then would he get married to her? One solution is to free and give her the choice of marriage. Do you want to get married to me or to go back to your people? However, going back to her people may not be a good decision because people who were against Muslims, and one will not so freely send them back to their people.
Sometimes, it seems that releasing them is an act of kindness or made exchange them for prisoners of war. Meaning, people who have been captured in war as one have captured her or him in war. A compromise would have been for prisoners to be held temporarily. If a woman was held in this situation temporarily, then contracting a permanent marriage between her and her master, perhaps this concubinage was seen as in-between where she could be temporarily married to her master. She's not a regarded as a full and permanent wife. However, she's given a temporary status as a wife, and it's called differently in the Quran. Yet, this needs to be explored in greater detail by discussing the ins and outs of the system. It could be used as a possible reference.