Sheikh Muhammad Iqbal Nadvi, Imam of Calgary Mosque, Canada, and Former Professor at King Saud Univ., Saudi Arabia, answers: This question is only of academic value now. It is most likely that people know about this issue theoretically, because some of the critics of Islam raise this question to attack Islam and its principles. The situation of having concubines is related to several things: First, if the Islamic state exists. Second, if the Islamic state makes offers for other territories to join Islam or enter into treaties with them. Third, if those territories refuse all kinds of peace and amicable offers, or if they announce war. Fourth, during the time of war, both sides capture prisoners that are exchanged mutually, then there’s no concubines. Fifth, if the prisoners have no possibility of being exchanged and they are kept under the conquered army, then the following things happen: Either they are killed, as what happened in Siberia, or they are put in prison where they are humiliated to death or the females are used as concubines.
Here it’s to be stressed that Islam has no double standards, and the situation of concubines is not a desirable option in Islam. If it happens, however, Islam solves it in a way akin to Islamic philosophy. That is, Islam deals with the issue on individual basis in the sense that the captives are distributed to Muslim individuals who can take care of them, teach them, and when they feel safe, free them. For this purpose, Islam related the freedom of slaves, with many Kaffarat (expiations) of sins.
Coming to your questions, there are two cases of concubine: one is a slave-girl living with a person as a maid only, in which case she will serve him, but he is not allowed to establish any sexual relationship with her. The other case is, if he decides to keep her as a partner, then he can establish a relationship with her, and then she will be freed as soon as she delivers any baby for him. Also, he will be the only person who has a relationship with her. This solution prevents any kind of prostitution, and at the same time, it finally leads towards the freedom of these concubines.
Currently, the conditions mentioned above do not exist, so no one can have concubines nowadays. That is why I consider the issue of concubines to be only of academic value, meaning that you only know it theoretically, because some of the critics of Islam raise this question to attack Islam.