“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.” (CCC 2351)
This is precisely the text I checked. And, as you show, CCC 2351 does
not say that lust is gravely disordered; only that it is disordered. This is a significant omission, because, when the Catechism discusses grave matter, it ordinarily
states that it is discussing a grave matter. Consider the other offenses considered in that very chapter:
2352: “…masturbation is an intrinsically and
gravely disordered action.”
2353: “Fornication… is
gravely contrary to the dignity of persons… Moreover, it is a
grave scandal when there is corruption of the young.”
2354: “Pornography… does
grave injury to the dignity of its participants.”
2355: “…it is always
gravely sinful to engage in prostitution…”
2356: “Rape… causes
grave damage that can mark the victim for life. It is
always an intrinsically evil act.”
2357: “Basing itself on Sacred Scripture, which presents homosexual acts as acts of
grave depravity, tradition has always declared that ‘homosexual acts are
intrinsically disordered.’”
2351 does
not state that lust is grave matter, and, considered in context, it seems that the CCC does
not consider it grave matter. It instead states only that lust is disordered, which is true of
all sin, including venial sin. I am not familiar with any doctrinal authority which
does state that lust is grave matter. (The AudioSanctio sermon you link is interesting and well-reasoned, but, outside its citation of Alexander VII on sensuous kissing, it is not a doctrinal authority.) This is the reason for my uncertainty on the gravity of lust.
I agree with you on the other principles you raise. You’re right that we mustn’t forget the possibility of improper sexual desire within a marriage!
EDIT: Ya’ll got me thinking, so I went theology-diving, and quickly came up with some Aquinas on this subject. He concludes that, while not all thoughts about fornication are sinful or mortally sinful, deliberate fantasy of the sort described in the OP does indeed constitute grave matter, and therefore a mortal sin if combined with full knowledge and full consent:
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.
That’s the good ol’ classic
Summa Theologiae, I-II, Q74, A8 (
link). I am persuaded of the point. I just wonder why the CCC did not mention that lust is
gravely wrong.
)