It's true that there are "internal acts" which are deliberate: indulging in lustful fantasies, using mental images for self-stimulation, generally running porn in your head. To the entent that they are intentional, these are sins in themselves, whether you ever act on them externally or not.
This is different, however, from unbidden temptations: feelings of desire which distract or torment you, and which you did not choose to harbor in your head. People troubled in this way (whether they are temptations to lust, wrath, jealousy, envy, revenge or whatever) have an obligation to figure out how to drain the power out of these thoughts, change them, or escape them without succumbing.
Bottom line: unbidden feelings are not sins, even if they assail you strongly and persistently. Inward surrender to lustful thoughts, though, is a sin.
I reckon 99% of people experience forbidden appetites at some point in their lives -- maybe for years ---- and have to learn how to achieve a level of inward peace and self-control.
This is a major and gradual achievement, a struggle, and not an easy one for most of us.