(a) Thus, for the sinner himself it is like a cruel goad that constantly annoys him and takes away his peace (St. Ambrose), like a sword that kills the nobler instincts (St. Gregory the Great), like a descent from human dignity to a condition below the beasts (St. Eusebius of Caesarea).
(b) For society it is disastrous in many ways, since it propagates dread mental and physical diseases, disrupts the peace of families, brings disgrace and destitution on innocent children, eats away fortunes and leads up to innumerable crimes of injustice and violence.
2496. Is Impurity Ever a Venial Sin?—(a) By reason of the imperfection of the act, impurity is venial when there is no sufficient deliberation or consent. Invincible ignorance in reference to the sixth commandment itself sometimes happens, especially in reference to internal sins of thought, to external sins of pollution if the person is young, and to other external sins when there is some complication of circumstances (e.g., kissing and other intimacy by engaged persons, onanism when married persons are poor or the woman sickly); and more frequently there is invincible ignorance about details of the sixth commandment (e.g., about the precise theological or moral malice of what is known to be sinful).
(b) By reason of the matter, impurity according to the common teaching is always mortal if directly willed, but sometimes venial if only indirectly willed. Impurity is directly willed when one posits an act intending to obtain from it unlawful venereal delectation, or perceives that such delectation is already present and consents to it. No matter how brief this voluntary assent, no matter how slight the commotion of the animal nature, no matter how far from the consummated is the impure act in question, there is always a serious injury done to a great good or at least (exception is made for the case of married persons) the proximate danger of such injury, and hence mortal sin (see 260). That even slight yielding to impurity is a serious peril is the teaching of Scripture (which declares that lust has killed even the strongest, Prov., vii. 26), of the Church (which condemns the opinion that libidinous kisses are not dangerous, see Denzinger, Enchiridion, n. 1140), of theology (which reminds us that by original sin reason has been darkened, the will enfeebled and the passions strengthened), and of experience (which shows that those who expose themselves to passion’s flame will be burnt). A small spark of fire is not trivial in the vicinity of a powder magazine, a minute flaw in a machine is not unimportant if it may bring on disaster, a first step is not safe if it is made on a slippery downward declivity.
(c) Impurity is indirectly willed when deliberately and without sufficient reason one posits an act which is not venereal pleasure (whether the act be good, such as a prayer made with great sensible fervor, or bad, such as gluttony, or indifferent, such as reading a book, looking at a picture, taking a bath), but which produces foreseen venereal pleasure (consummated or non-consummated) that one neither intends nor directly consents to. Impurity thus indirectly willed is sinful, because the pleasure is foreseen and permitted without sufficient reason (see 102), or in other words because one exposes oneself to danger of internal defilement (consent), or external pollution without justification (see 260). Indirect impurity is mortal when there is proximate danger of grave sin in the act done, that is, when the posited act _per se_ or from its nature strongly incites the agent to sexual passion, as when one gazes long and fixedly at obscene pictures, knowing that always or nearly always this arouses impure emotions. The sin is venial when there is only remote danger of grave sin. This happens when the posited act is not of a venereal kind (an unnecessary conversation on indifferent topics) or is only mildly exciting (e.g., a passing glance at an obscene object), or when the agent himself is not greatly affected by it (e.g., when an old man, or one who is of very cold disposition, or an artist whose only thought is the esthetic excellence, carefully studies a picture of the nude).
2497. Temptations to Impurity.—Before treating the various kinds of impurity, we shall speak briefly of temptations that occasion this sin and of the duties of the person tempted.
(a) External temptation comes from the devil or the world, and the duty of struggling against it has been treated elsewhere (see 252, 1455 sqq., 1495 sqq.). Thus, he who finds that certain persons, places or things are for him a temptation to impurity must be guided by the principles given for occasions of sin (263 sqq.); he who finds that another wishes to seduce him into impurity must refuse all internal consent (see 254 sqq.), and must also resist violence when there is hope of success, or when this is necessary to avoid giving scandal or yielding consent (see on self-defense, 1841).
(b) Internal temptation comes from the flesh. It consists in inchoative disturbances or excitements of the organs or fluids that serve generation (e.g., erections, clitoral movements). Sometimes it is produced involuntarily, without any intention or consent of the will, by physiological states (e.g., conditions of the blood, nerves, etc., due solely to the weather, to disease, to aphrodisiac properties of ailment, to clothing, or position) or by psychical states (e.g., spontaneous images or appetites of the soul mentioned in 129), and in these cases the temptation is manifestly free from all sin. St. Pius V condemned the teaching of Baius that those who suffer motions of concupiscence against their will are transgressors of the command: “Thou shalt not covet” (see Denziger, Enchiridion, nn. 1050, 1051, 1075). Sometimes the temptation is directly voluntary, as when the passion is deliberately awakened for the purpose of sin, and then there is grave guilt (see 2496 b). Sometimes the temptation is indirectly voluntary, as when with the foresight of the passion but without desire of it an action is performed that arouses it. In this last case, if there is a just reason for the excitatory action (e.g., a physician sees and hears things that are calculated to be a temptation, but his reason is the exercise of his profession), no sin is committed; but if there is no just reason for the action (e.g., a person reads an erotic book, and curiosity is his only motive), sin is committed, and its gravity depends on the amount of danger to which one exposes oneself (see 2496 c).
2498. Resistance to Internal Temptations.—The fight against internal temptations is of various kinds.
(a) By reason of its subject, the conflict is chiefly in the will, to which it belongs to give or withhold consent; secondarily, in the other powers of the soul and the body, which under command from the will perform acts designed to overcome temptation.