Objection 1. It would seem that consent to delectation is not a mortal sin, for consent to delectation belongs to the lower reason, which does not consider the eternal types, i.e. the eternal law, and consequently does not turn away from them. Now every mortal sin consists in turning away from Augustine's definition of mortal sin, which was quoted above (Question 71, Article 6). Therefore consent to delectation is not a mortal sin.
Objection 2. Further, consent to a thing is not evil, unless the thing to which consent is given be evil. Now "the cause of anything being such is yet more so," or at any rate not less. Consequently the thing to which a man consents cannot be a lesser evil than his consent. But delectation without deed is not a mortal sin, but only a venial sin. Therefore neither is the consent to the delectation a mortal sin.
Objection 3. Further, delectations differ in goodness and malice, according to the difference of the deeds, as the Philosopher states (Ethic. x, 3,5). Now the inward thought is one thing, and the outward deed, e.g. fornication, is another. Therefore the delectation consequent to the act of inward thought, differs in goodness and malice from the pleasure of fornication, as much as the inward thought differs from the outward deed; and consequently there is a like difference of consent on either hand. But the inward thought is not a mortal sin, nor is the consent to that thought: and therefore neither is the consent to the delectation.
Objection 4. Further, the external act of fornication or adultery is a mortal sin, not by reason of the delectation, since this is found also in the marriage act, but by reason of an inordinateness in the act itself. Now he that consents to the delectation does not, for this reason, consent to the inordinateness of the act. Therefore he seems not to sin mortally.
Objection 5. Further, the sin of murder is more grievous than simple fornication. Now it is not a mortal sin to consent to the delectation resulting from the thought of murder. Much less therefore is it a mortal sin to consent to the delectation resulting from the thought of fornication.
Objection 6. Further, the Lord's prayer is recited every day for the remission of venial sins, as Augustine asserts (Enchiridion lxxviii). Now Augustine teaches that consent to delectation may be driven away by means of the Lord's Prayer: for he says (De Trin. xii, 12) that "this sin is much less grievous than if it be decided to fulfil it by deed: wherefore we ought to ask pardon for such thoughts also, and we should strike our breasts and say: 'Forgive us our trespasses.'" Therefore consent to delectation is a venial sin.
On the contrary, Augustine adds after a few words: "Man will be altogether lost unless, through the grace of the Mediator, he be forgiven those things which are deemed mere sins of thought, since without the will to do them, he desires nevertheless to enjoy them." But no man is lost except through mortal sin. Therefore consent to delectation is a mortal sin.
I answer that, There have been various opinions on this point, for some have held that consent to delectation is not a mortal sin, but only a venial sin, while others have held it to be a mortal sin, and this opinion is more common and more probable. For we must take note that since every delectation results from some action, as stated in Ethic. x, 4, and again, that since every delectation may be compared to two things, viz. to the operation from which it results, and to the object in which a person takes delight. Now it happens that an action, just as a thing, is an object of delectation, because the action itself can be considered as a good and an end, in which the person who delights in it, rests. Sometimes the action itself, which results in delectation, is the object of delectation, in so far as the appetitive power, to which it belongs to take delight in anything, is brought to bear on the action itself as a good: for instance, when a man thinks and delights in his thought, in so far as his thought pleases him; while at other times the delight consequent to an action, e.g. a thought, has for its object another action, as being the object of his thought; and then his thought proceeds from the inclination of the appetite, not indeed to the thought, but to the action thought of. Accordingly a man who is thinking of fornication, may delight in either of two things: first, in the thought itself, secondly, in the fornication thought of. Now the delectation in the thought itself results from the inclination of the appetite to the thought; and the thought itself is not in itself a mortal sin; sometimes indeed it is only a venial sin, as when a man thinks of such a thing for no purpose; and sometimes it is no sin at all, as when a man has a purpose in thinking of it; for instance, he may wish to preach or dispute about it. Consequently such affection or delectation in respect of the thought of fornication is not a mortal sin in virtue of its genus, but is sometimes a venial sin and sometimes no sin at all: wherefore neither is it a mortal sin to consent to such a thought. In this sense the first opinion is true.
But that a man in thinking of fornication takes pleasure in the act thought of, is due to his desire being inclined to this act. Wherefore the fact that a man consents to such a delectation, amounts to nothing less than a consent to the inclination of his appetite to fornication: for no man takes pleasure except in that which is in conformity with his appetite. Now it is a mortal sin, if a man deliberately chooses that his appetite be conformed to what is in itself a mortal sin. Wherefore such a consent to delectation in a mortal sin, is itself a mortal sin, as the second opinion maintains.
Reply to Objection 1. Consent to delectation may be not only in the lower reason, but also in the higher reason, as stated above (Article 7). Nevertheless the lower reason may turn away from the eternal types, for, though it is not intent on them, as regulating according to them, which is proper to the higher reason, yet, it is intent on them, as being regulated according to them: and by turning from them in this sense, it may sin mortally; since even the acts of the lower powers and of the external members may be mortal sins, in so far as the direction of the higher reason fails in directing them according to the eternal types.
Reply to Objection 2. Consent to a sin that is venial in its genus, is itself a venial sin, and accordingly one may conclude that the consent to take pleasure in a useless thought about fornication, is a venial sin. But delectation in the act itself of fornication is, in its genus, a mortal sin: and that it be a venial sin before the consent is given, is accidental, viz. on account of the incompleteness of the act: which incompleteness ceases when the deliberate consent has been given, so that therefore it has its complete nature and is a mortal sin.
Reply to Objection 3. This argument considers the delectation which has the thought for its object.
Reply to Objection 4. The delectation which has an external act for its object, cannot be without complacency in the external act as such, even though there be no decision to fulfil it, on account of the prohibition of some higher authority: wherefore the act is inordinate, and consequently the delectation will be inordinate also.
Reply to Objection 5. The consent to delectation, resulting from complacency in an act of murder thought of, is a mortal sin also: but not the consent to delectation resulting from complacency in the thought of murder.
Reply to Objection 6. The Lord's Prayer is to be said in order that we may be preserved not only from venial sin, but also from mortal sin.
Source: Summa Theologica (New Advent)