Before we can ask whether there is anything morally wrong with lust we have to know what we are talking about. What is lust? Here is a start:
The inordinate craving for, or indulgence in, the carnal pleasure which is experienced in the human organs of generation.
But this won't do as it stands since it mixes desire and satisfaction in the same definition. It also fails to distinguish between lust as an occurrent state and lust as a disposition or propensity. Suppose we distinguish:
1. Desire for sexual pleasure
2. Inordinate desire for sexual pleasure
3. Satisfaction of the desire for sexual pleasure
4. Satisfaction of the inordinate desire for sexual pleasure
5. Habitual satisfaction of the inordinate desire for sexual pleasure.
Virtues and vices are habits. Habits are dispositions of agents. As dispositions, virtues and vices can exist unexercised. Agents are persons. So virtues and vices are properly and primarily attributed to persons. But a secondary mode of speech is allowable: lustful or lecherous acts (whether types or tokens) are such in virtue of their being the acts of persons who are lustful or lecherous in the primary sense.
If lust is a vice, then it is a habit, and (5) appears adequate as a definition. We can then define a lecher as one whose characteristic vice is lust, just as a glutton is one whose characteristic vice is gluttony and a miser is one whose characteristic vice is avarice.
Thus we may assign lust to the category of habits. It is something dispositional in nature. The lustful person is disposed to satisfy inordinately his or her desire for sexual pleasure. 'Inordinate' is a normative term in that it implies that there is a proper or correct ordering of sexual desire.
But a habit need not be a vice. A habit could be a virtue or neither a virtue nor a vice. There are morally indifferent habits, e.g., the habit of shaving after showering, and not vice versa. Presumably, lust is a vice if it is a habit that vitiates, or weakens. Does lust weaken? Distinguish physical from moral weakening. The exercise of lust needn't physically weaken, except temporarily; but it arguably does morally weaken inasmuch as it makes it more difficult to control the appetites generally. The 'rational part' then gets swamped and suborned -- which can't be good. But at the moment I am mainly concerned just to define lust, not to condemn it.
Is a vice a sin? Sin is a religious concept. One cannot properly speak of sin outside the context of religion. Indeed, it seems one cannot properly speak of sin outside the context of theistic religion. Not every religion is theistic. Or are there sins in Buddhism? In a slogan: no God, no sin. But even if all religion is either false or meaningless, virtue ethics can still be a going enterprise. So I suggest that we not conflate the concepts of vice and sin. The fact that 'sin' can be used and is sometimes used to refer to any old transgression of any old rule, as in talk of 'sins against logic,' proves nothing.
Vices vitiate while virtues empower. Vices are weaknesses while virtues are strengths. But there has to be more to it than that because of the normative element.
'Lust' can be used to refer to strong desire or craving. But this is an extended use of the word. Thus if I say that Hillary lustsafter power, I am using 'lust' in an extended or analogous way: I am not suggesting that Hillary's desire for power is sexual in nature. There is nothing wrong with extended uses of terms as long as one realizes what one is doing. There is nothing wrong with speaking of a lust for money so long as you realize that that way of talking gives no aid and comfort to the notion that avarice is a species of lust.
Ad 1. Lust is not desire for sexual pleasure. The latter is both natural and morally unobjectionable. Lust, however, is morally objectionable. (Yes, I know I haven't proved this. But can it be proved? From which premises? And can they be proved?)
Ad 2. To be lustful, a sexual desire must be inordinate. This is a normative term, obviously. An inordinate desire is one that exceeds what is right and proper. It is not just a powerful desire, or a desire that is excessive in some nonnormative sense. Now suppose I have a powerful, and indeed an inordinate, desire for sexual pleasure, but I resist the desire. Strictly speaking, I am not lustful. Lust is morally objectionable, but my resistance to inclination is morally praiseworthy.
You say this goes against ordinary usage? Then I say so much the worse for ordinary usage! My concern is not to define words of ordinary language, but to delimit a phenomenon. You might say I am doing moral phenomenology. I am trying to capture the essence of a certain deleterious propensity widespread among human beings. I am not tied to the apron strings of ordinary language.
I am saying: Look at this phenomenon. How can we best describe its essence? I am not primarily interested in how 'lust' is most often used in ordinary English. Ordinary language has no veto-power over philosophical results. Appeals to ordinary language cut no ice in serious philosophy. This is not to say one can ignore ordinary language. Sifting through ordinary usage is often an indispensable proto-philosophical exercise.
Ad 3 and 4. Lust must therefore involve the satisfaction of inordinate sexual desire. But even this is not enough. Someone who satisfies his inordinate sexual desire once or a few times is no more a lecher than one who overeats once or a few times is a glutton. Similarly, one who pursues an exercise regimen for a week and then relapses into sloth is still a couch potato.
Ad 5. The satisfaction must be habitual. Lust is therefore a habit, and indeed a vice. It is a disposition to behave in a certain way. As such, it can exist even when unexercised. A lustful man is lustful even when he is sated or sleeping. A lustful thought or deed is lustful because its springs from a lustful character.