Objection 1. It would seem that it has been sometimes lawful to have a concubine. For just as the natural law requires a man to have but one wife, so does it forbid him to have a concubine. Yet at times it has been lawful to have several wives. Therefore it has also been lawful to have a concubine.
Objection 2. Further, a woman cannot be at the same time a slave and a wife; wherefore according to the Law (Deuteronomy 21:11, seqq.) a bondswoman gained her freedom by the very fact of being taken in marriage. Now we read that certain men who were most beloved of God, for instance Abraham and Jacob, had intercourse with their bondswomen. Therefore these were not wives, and consequently it was sometime lawful to have a concubine.
Objection 3. Further, a woman who is taken in marriage cannot be cast out, and her son should have a share in the inheritance. Yet Abraham sent Agar away, and her son was not his heir (Genesis 21:14). Therefore she was not Abraham's wife.
On the contrary, Things opposed to the precepts of the decalogue were never lawful. Now to have a concubine is against a precept of the decalogue, namely, "Thou shalt not commit adultery." Therefore it was never lawful.
Further, Ambrose says in his book on the patriarchs (De Abraham i, 4): "What is unlawful to a wife is unlawful to a husband." But it is never lawful for a wife to put aside her own husband and have intercourse with another man. Therefore it was never lawful for a husband to have a concubine.
I answer that, Rabbi Moses says (Doc. Perp. iii, 49) that before the time of the Law fornication was not a sin; and he proved his assertion from the fact that Juda had intercourse with Thamar. But this argument is not conclusive. For there is no need to excuse Jacob's sons from mortal sin, since they were accused to their father of a most wicked crime (Genesis 37:2), and consented to kill Joseph and to sell him. Wherefore we must say that since it is against the natural law to have a concubine outside wedlock, as stated above (Article 3), it was never lawful either in itself or by dispensation. For as we have shown (Doc. Perp. iii, 49) intercourse with a woman outside wedlock is an action improportionate to the good of the offspring which is the principal end of marriage: and consequently it is against the first precepts of the natural law which admit of no dispensation. Hence wherever in the Old Testament we read of concubines being taken by such men as we ought to excuse from mortal sin, we must needs understand them to have been taken in marriage, and yet to have been called concubines, because they had something of the character of a wife and something of the character of a concubine. In so far as marriage is directed to its principal end, which is the good of the offspring, the union of wife and husband is indissoluble or at least of a lasting nature, as shown above (Article 1), and in regard to this there is no dispensation. But in regard to the secondary end, which is the management of the household and community of works, the wife is united to the husband as his mate: and this was lacking in those who were known as concubines. For in this respect a dispensation was possible, since it is the secondary end of marriage. And from this point of view they bore some resemblance to concubines, and for this reason they were known as such.
Reply to Objection 1. As stated above (1, ad 7,8) to have several wives is not against the first precepts of the natural law, as it is to have a concubine; wherefore the argument does not prove.
Reply to Objection 2. The patriarchs of old by virtue of the dispensation which allowed them several wives, approached their bondswomen with the disposition of a husband towards his wife. For these women were wives as to the principal and first end of marriage, but not as to the other union which regards the secondary end, to which bondage is opposed since a woman cannot be at once mate and slave.
Reply to Objection 3. As in the Mosaic law it was allowable by dispensation to grant a bill of divorce in order to avoid wife-murder (as we shall state further on, 67, 6), so by the same dispensation Abraham was allowed to send Agar away, in order to signify the mystery which the Apostle explains (Galatians 4:22, seqq.). Again, that this son did not inherit belongs to the mystery, as explained in the same place. Even so Esau, the son of a free woman, did not inherit (Romans 9:13, seqq.). In like manner on account of the mystery it came about that the sons of Jacob born of bond and free women inherited, as Augustine says (Tract. xi in Joan.) because "sons and heirs are born to Christ both of good ministers denoted by the free woman and of evil ministers denoted by the bondswoman."
Source: Summa Theologica (New Advent)