The sin of lust is an act whereby the human person interiorly consents to the disordered desire for a sexual act that he knows to be gravely immoral, such as adultery. In this sin, the person is truly willing to commit the exterior act, if there were an opportunity. But this interior act is substantially different from merely fantasizing about a sexual act. Lust is essentially consent to the sinful sexual act itself (though in desire), not merely consent to thoughts about the act.
[Matthew]
{5:27} You have heard that it was said to the ancients: ‘You shall not commit adultery.’
{5:28} But I say to you, that anyone who will have looked at a woman, so as to lust after her, has already committed adultery with her in his heart.
{5:29} And if your right eye causes you to sin, root it out and cast it away from you. For it is better for you that one of your members perish, than that your whole body be cast into Hell.
Jesus condemns both the exterior act of adultery, and the interior act of lust. The exterior act of merely looking at a woman is not a sin. Admiring beauty in a woman is not adultery of the heart. Only if the man consents to lust with the will and intellect does he commit adultery of the heart. Jesus describes lust as adultery of the heart because any act of lust is a willingness to commit an illicit sexual act, including any gravely immoral sexual act, the primary example of which is adultery. If the person is not willing to commit the immoral sexual act, then even if there is some degree or type of sin, the sin is not lust.
Saint Thomas Aquinas: "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 [enjoyment] 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…” Summa Theologica, I-II, Q. 74, n. 8.
Consent to thoughts about sexual acts is not necessarily a mortal sin. And passing thoughts to which one does not consent would not be sins at all.
Spiro…
(I can not recommend the above post or the links provided for somethings there are incorrect…)
From the Catechism:
2351 Lust is disordered desire for or inordinate enjoyment of sexual pleasure. Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes.
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A person can lust even in his own thoughts…even without intending to do any exterior act…even if he is unwilling to commit an actual exterior act.
and of course a person can lust while doing exterior acts …
of course remember …often times it can happen that some thought or sexual pleasure comes…and is not consented to…or wanted…
It can be that does not consent to any sin but actually preforming an act of the virtue of chastity…and growing perhaps in this virtue…
it is one thing for an unintended and not consented to sexual feeling to happen to one…outside of marriage…
it is another to seek or consent to such (thus being lust).
of course too…it can happen that even in grave matters…where sexual pleasure involved…but which do not have full knowledge at the time…or where there is only partial consent…such would by definition be a venial sin.
Compendium of the Catechism:
- When does one commit a mortal sin?
1855-1861
1874
One commits a mortal sin when there are simultaneously present: grave matter, full knowledge, and deliberate consent. This sin destroys charity in us, deprives us of sanctifying grace, and, if unrepented, leads us to the eternal death of hell. It can be forgiven in the ordinary way by means of the sacraments of Baptism and of Penance or Reconciliation.
- When does one commit a venial sin?
1862-1864
1875
One commits a venial sin, which is essentially different from a mortal sin, when the matter involved is less serious or, even if it is grave, when full knowledge or complete consent are absent. Venial sin does not break the covenant with God but it weakens charity and manifests a disordered affection for created goods. It impedes the progress of a soul in the exercise of the virtues and in the practice of moral good. It merits temporal punishment which purifies.