Most legalists* held one of two opinions: 1) a concubine was a “one man harlot”** and both parties were guilty of fornication or 2) a man and his concubine should be considered spouses who had married clandestinely and neither party could be considered to have sinned unless they ended the union. Both of these groups were working from the idea that both non-procreative and non-marital sex were entirely sinful. The one, however, took the route of flat out condemnation, while the other attempted to modify the definition of concubinage to prove that it was really marriage.
Adherents of the first view looked primarily to the opinions of Saints Paul and Augustine. Both men held that only two moral options were open to Christians: marriage or celibacy. Concubinage, not being marriage, was therefore entirely immoral and had no place in Christian society.
Such a well-established tradition, however, could not so easily be eradicated and many legalists found it easier to compromise by declaring marriage and concubinage to be essentially the same thing. Gratian, for example, held that partners in concubinage were bound together by marital affection and therefore should be considered to have married informally. Such a designation, however, necessitated qualification. If one were to declare concubinage to be marriage, any practices that were permissible in the former relationship that were not in the latter must then be discarded as unacceptable. The same term was still used for both acceptable and unacceptable relationships. So an informal union that was intended to be permanent and a temporary informal union were lumped under the same category of “concubinage” despite one being permissible and the other entirely sinful.
These opinions remained on a relatively equal footing, the one supported by the stringent morality of certain early Church Fathers, the other by longstanding common practice, up until the Reformation, when the stricter interpretation gained ascendance. The Council of Trent banned concubinage entirely in the 16th century.
*Of those whose work we still have.
**Which then makes you wonder how they’re defining “harlot.”
Brundage, James A. Law, Sex, and Christian Society in Medieval Europe. Chicago: University of Chicago Press, 1987.
Brundage, James A. "Concubinage and Marriage in Medieval Canon Law." Journal of Medieval History (1975): 1-17.
Dübeck, Inger. "Women, Weddings and Concubines in Medieval Danish Law." Scandinavian Journal of History 17.4 (1992): 315-322.
Marriage Canons in The Decretum of Gratian - Catholic University of America