FORNICATION. — From the Latin *fornix*, which designated a vaulted chamber or corridor, usually constructed as a support for staircases (in stadia, theaters, amphitheaters) and adapted for public use or even as a gathering place for prostitutes. Christianity forged the terms *fornicarius*, *fornicatio*, which became customary for indicating any carnal commerce outside legitimate marriage. In this sense, they often recall in the Vulgate equivalents of the Hebrew term *zēmūnīm*, which in its proper sense meant sexual impurity, especially adultery, and in a metaphorical sense idolatry, as a violation of the nuptial bond between Israel and God.
An example of the first sense is found in the Gospel of Matthew (5:32; 19:9), where the term "fornicatio" in the phrase "excepta fornicationis causa" means adultery; an example of the second sense is found in the Gospel of John (8:41). The word acquired a technical meaning, so that among medieval theologians the term *fornicatio* designated that particular kind of lust consisting in sexual union between two persons free from marital bonds or other ties that would confer upon their sin the qualification of sacrilege or incest. This meaning has remained in the technical vocabulary of Catholic moral theology.
The main question raised regarding fornication in moral theology concerns its illicit nature. Greco-Roman civilization, while condemning adultery and incest, did not regard simple fornication as a moral fault, except when it involved shameless license. Christianity immediately considered fornication illicit; the Apostolic Council of Jerusalem expressly forbade it to neoconvertite Christians from paganism (Acts 15:20); St. Paul declares that fornicators, along with adulterers, are excluded from the Kingdom of God (1 Cor 6:9). The patristic tradition unanimously maintained the apostolic line of thought. Medieval theological speculation sought to determine the intrinsic reason and degree of immorality in fornication; with very rare exceptions (Durandus, Caramuel, and a few others), theologians agree in considering fornication intrinsically and gravely illicit, basing their judgment both on revealed data and on reasons deduced from the analysis of the objective content of fornication.
These reasons, repeatedly expounded by St. Thomas in his works (Summa Theol., II-II, q. 154, a. 2; Contra Gentiles, III, cap. 122; De Malo, q. 15, aa. 1-2), locate the intrinsic illicitness of fornication in its lack of the legal stability required by the purpose of continuous commerce, which, as it includes not only procreation but also the education of offspring. The essential words of the Angelic Doctor are quoted: “It is already seen in the animal kingdom that, where the rearing of offspring requires the cooperation of male and female, the two unite in a determined and stable form, as, for example, is observed in birds... Now it is evident that for the education of a human being not only the mother’s work of nourishing is required, but much more the father’s work of instructing and defending, and further developing him both spiritually and materially. It is therefore contrary to nature that the conjugal union be made independently of every bond, but it is necessary that this occur between determined and stable persons, while bound by a bond. Moreover, the natural need that the male in the human species feels to recognize with certainty who his children are is intimately connected with the duty incumbent upon him to educate them. Now this certainty would not be possible if the conjugal union were free from every bond. Therefore nature itself postulates that the conjugal union occur between two determined and identifiable persons; that is, the conjugal act requires the marital state. And since the marital state, having as its purpose procreation, is of the utmost importance to the common good, it is subject to law. Fornication, therefore, as extramarital union outside the law, is intrinsically sinful. Nor is it valid to object that someone might be willing to assume the education of offspring conceived through fornication; because the natural necessity of binding the conjugal union to the legally established marital state, arising from reasons of a universal character, precedes and comprehends all particular cases” (Summa Theol., loc. cit.). St. Thomas, and with him all Catholic moralists in agreement, declare fornication a grave sin from the objective point of view, while acknowledging how widespread and easy is the erroneous subjective evaluation to the contrary in environments ignorant of and hostile to Christianity. The intrinsic reason adduced is drawn from the grave harm resulting, on the universal plane—even if not always in every particular case—to the offspring born of a fornication union. The supreme magisterium has expressly pronounced on the intrinsic illicitness of fornication, condemning through the mouth of Pope Innocent XI the following proposition: “It seems clearly evident that fornication in itself involves no malice and is evil only because it is forbidden, so that it appears altogether discordant with reason” (Denz.-U., 1198). The encyclicals of Leo XIII (*Arcanum*) and Pius XI (*Casti connubii*) on Christian marriage authoritatively reaffirm the traditional doctrine set forth above regarding the illicitness of any extramarital sexual commerce (cf. FREE LOVE; CONCUBINAGE; MARRIAGE; PROSTITUTION).