Many Christians refuse to acknowledge lust (and everything associated with it) as sin because they don't want to feel bad about the lust they experience.
On the contrary, they want to feel good about it, and the best way to do that it is to redfine lust as not-sin.
Not-sin means no repentance required. Yay! Problem solved!
Not so fast...
Over at From the Narrow Desert, Wm Jas Tychonievich offers an incisive explanation of what repentance is and why it is so crucial (bold and slight editing added):
True repentance is not an emotion any more than the pure love of Christ is an emotion, and obviously repentance doesn't mean never doing anything bad ever again. God would have designed this world rather differently if that was what he expected!
What is repentance, then? My current understanding is that repentance is confession. No, not confession in the sense of saying, "Bless me, Father, for I have committed adultery in my heart 700 times this week." Confession doesn't mean rattling off an itemized list of one's recent misdeeds to God or a priest.
It means acknowledging sin as sin.
The unrepentant are those who make excuses for themselves, who deny that their sins are sins and are therefore unwilling to give them up. Willingness is all; the flesh is weak, but the spirit must be willing.
Daily repentance does not mean daily groveling for forgiveness like a beaten dog; it means reminding oneself what is good and what is evil, what is of God and what is not, and then going on with life, confident in the knowledge that "he that believeth is not condemned."
And yes, of course we should try to be virtuous and to sin less, but in the end no such projects can really succeed in this present life.
They are not what repentance is, and they are not that on which salvation depends.
Read the rest of Wm's lucid thoughts on repentance here.