The Lord's Sermon on the Mount shows the fulfillment of the law.  In its course, Jesus gives examples of this fulfillment: first, by the fulfillment of the fifth commandment, regarding murder; and then by the six commandment, regarding adultery.  In his commentary on The Sermon on the Mount, St. Augustine of Hippo makes an important distinction when it comes to defining the sin of lust:

"It is a lesser righteousness to not commit adultery in the joining of the flesh, but the greater righteousness of the kingdom of God is not to commit adultery in the heart.  He who does not commit adultery in his heart much more easily guards his heart lest he commit adultery in his flesh.  He who gave the first commandment, confirmed the second, for he did not come to destroy the law but to fulfill it.  It must be reasonably considered that he did not say that everyone who strongly desires a woman commits adultery in his heart, but that he who looks at a woman in order to lust after her, does so.  That is, he who turns to a woman with this end and intention in mind -- to lust after her.  Lust is not simply arousal for fleshly delight, but is a full consent to pleasure so that the unlawful desire is not restrained, but would be satisfied if the opportunity presented itself. "