Pope St. John Paul II
“Christ’s words are demanding. They demand that in the sphere in which relationships with persons of the other sex are formed, man has full and deep conscious of his own acts, and above all in his interior acts, and that he is consciousness of the inner impulses of his own ‘heart’ so that he can identify and evaluate them in a mature way. Christ’s words demand that in this sphere, which seems to belong only to the body and senses (that is, to the exterior man), he should succeed in being really an interior man, able to obey right conscious, able to be the authentic master of his own innermost impulses, like a watchman who watches over a hidden spring, and finally able to draw from all these impulses what is fitting for ‘purity of heart’ by building with consciousness and consistency the personal sense of the spousal meaning of the body, which opens the interior space of the freedom of the gift.”
Pope John Paul II, Man and Woman He Created Them, Pauline Books and Media, 2006.