The heart has become a battlefield between love and lust. The more lust dominates the heart, the less the heart experiences the nuptial meaning of the body. It becomes less sensitive to the gift of the person, which expresses that meaning in the mutual relations of man and woman. Certainly, that lust which Christ speaks of in Matthew 5:27-28 appears in many forms in the human heart. It is not always plain and obvious. Sometimes it is concealed, so that it passes itself off as love, although it changes its true profile and dims the limpidity of the gift in the mutual relationship of persons. Does this mean that it is our duty to distrust the human heart? No! It only means that we must keep it under control.

Pope John Paul II, Theology of the Body (General Audience, July 23, 1980)