2520. The Consummated Sins of Impurity.—There are in all seven species of completed acts of impurity. (a) Thus, some sins of impurity are against reason because they do not observe the ends of sexual intercourse. These ends are, first, the begetting of children (to which is opposed unnatural impurity), and, secondly, the rearing of children (to which is opposed fornication).
(b) Other sins of impurity are against reason because they violate a right of the person with whom intercourse is had (incest), or of a third party to whom that person belongs. If the third party is injured in conjugal rights, there is adultery; if in parental rights, there is defloration or rape, according as the injury is done without or with force; if in religious rights, there is sacrilege. This second category of sins is classed under impurity rather than under injustice, because the purpose of the guilty person and his act belong to venereal sin.
2521. Comparative Malice of the Sins of Consummated Lust.—(a) In the abuse of an act, the worst evil is the disregard of what nature itself determines as the fundamentals upon which all else depends, just as in speculative matters the worst error is that which goes astray about first principles. Now, the prime dictates of nature as to sexual intercourse are that it serve the race and the family. Hence, the sin of unnatural lust (which injures the race by defeating its propagation) and the sin of incest (which injures the family by offending piety) are the worst of carnal vices.
(b) In the abuse of an act a lesser evil is that which observes the natural fundamentals, but disregards what right reason teaches about things secondary, in the manner of performing the act. But reason requires that in sexual intercourse the rights of the individual be respected. A most serious violation of individual right is adultery, which usurps the right of intercourse belonging to another; next in gravity is rape, which violently seizes for lust a person under the care of another or undefiled; next is defloration, which trespasses on the right of guardianship, or removes bodily virginity, but without violence; last among these sins is fornication, which is an injury done not to the living, but to the unborn.
2522. Multiplication of Sins of Lust.—The various kinds of lust may be combined in one and the same act, as when unnatural vice (e.g., sodomy) is practised with a relative (incest). Sacrilege, of course, aggravates every other kind of carnal sin, and thus there is sacrilegious sodomy, sacrilegious adultery, sacrilegious incest, etc.
2523. Fornication.—Fornication is the copulation of an unmarried man with an unmarried woman who is not a virgin.
(a) It is copulation, or sexual intercourse suited for generation of children. Thus, it differs from lewdness, which consists in unconsummated acts, and from sodomitic intercourse, which is consummated but unsuited for generation. Onanism is an aggravating circumstance of fornication, or rather a new sin of unnatural intercourse. (b) It is committed by unmarried persons, and thus it differs from adultery. (c) It is committed with a woman, and is thus distinguished from sodomy. (d) It is committed with a woman who is not a virgin, and thus differs from defloration.
2524. Sinfulness of Fornication.—It is of faith that fornication is a mortal sin.
(a) Thus, it is gravely forbidden by the divine positive law. Hence, whores and whoremongers are an abomination to the Lord (Deut., xxiii. 17); fornicators are worthy of death (Rom., i. 29-32), they shall not enter the kingdom of God (Gal., V. 19-21; Eph., v. 5; Heb., xiii. 4; Apoc., xxi. 8). The Fathers teach that fornication is a grave crime (St. Fulgentius), and that it brings condemnation on the guilty person (St. Chrysostom). The declarations of the Church on the evil of this sin are found in the Council of Vienne and in the censures of Alexander VII and Innocent XI (Denzinger, nn. 477, 1125, 1198).
(b) Fornication is gravely forbidden by the natural law. For it is seriously against reason to cause an injury to the entire life of another human being; but fornication does this very thing by depriving the unborn child of its natural rights to legitimacy, to the protection of both parents, and to education in the home circle. True, in some cases there may be no prospect of a child, or there may be provision for its proper rearing; but these cases are the exception, since fornication from its nature tends to the neglect of the child, and the morality of acts must be judged, not by the exceptional and accidental, but by the usual and natural. Those who commit fornication are thinking of their own pleasure rather than of duty, and will generally shirk the difficult burdens of parenthood. Society also would be gravely wounded if unmarried intercourse were at any time lawful. Hence, St. Paul reproves the pagans, though ignorant of Scripture, for their sins of fornication (I Cor., vi. 9-11; Eph., v. 1-6), since reason itself should have taught them the unlawfulness of this practice. It seems, though, that invincible ignorance of the wrong of fornication is possible among very rude or barbarous people, since the injury to the neighbor does not show itself so clearly in this sin as in many others.