The inordinate appetite for sexual gratification, one of the seven deadly or capital vices.
God has placed in the organs of generation a sensual pleasure in order to induce man to do what is necessary for the preservation of the race.
To seek this pleasure, or consent to it, outside of matrimony, or contrary to the rules of married life, is siaful because it is contrary to the order established by God.
Even to lust with the eyes, or heart, is already a sin, as Our Lord teaches in the sermon on the Mount.
New Catholic Dictionary