What Christ himself taught
Concerning chastity, He uttered such sentiments as these: “Whosoever looks upon a woman to lust after her, has committed adultery with her already in his heart before God.” And, “If your right eye offend you, cut it out; for it is better for you to enter into the kingdom of heaven with one eye, than, having two eyes, to be cast into everlasting fire.” And, “Whosoever shall marry her that is divorced from another husband, commits adultery.” And, “There are some who have been made eunuchs of men, and some who were born eunuchs, and some who have made themselves eunuchs for the kingdom of heaven's sake; but all cannot receive this saying.” So that all who, by human law, are twice married, are in the eye of our Master sinners, and those who look upon a woman to lust after her.
For not only he who in act commits adultery is rejected by Him, but also he who desires to commit adultery: since not only our works, but also our thoughts, are open before God. And many, both men and women, who have been Christ's disciples from childhood, remain pure at the age of sixty or seventy years; and I boast that I could produce such from every race of men. For what shall I say, too, of the countless multitude of those who have reformed intemperate habits, and learned these things?
For Christ called not the just nor the chaste to repentance, but the ungodly, and the licentious, and the unjust; His words being, “I came not to call the righteous, but sinners to repentance.” For the heavenly Father desires rather the repentance than the punishment of the sinner. And of our love to all, He taught thus: “If you love them that love you, what new thing are you doing? For even fornicators do this. But I say unto you, Pray for your enemies, and love them that hate you, and bless them that curse you, and pray for them that despitefully use you.” And that we should communicate to the needy, and do nothing for glory, He said, “Give to him that asks, and from him that would borrow turn not away; for if you lend to them of whom you hope to receive, what new thing are you doing?
Even the publicans do this. Lay not up for yourselves treasure upon earth, where moth and rust does corrupt, and where robbers break through; but lay up for yourselves treasure in heaven, where neither moth nor rust does corrupt. For what is a man profited, if he shall gain the whole world, and lose his own soul? Or what shall a man give in exchange for it? Lay up treasure, therefore, in heaven, where neither moth nor rust does corrupt.” And, “Be kind and merciful, as your Father also is kind and merciful, and makes His sun to rise on sinners, and the righteous, and the wicked. Take no thought what you shall eat, or what you shall put on: are you not better than the birds and the beasts?
And God feeds them. Take no thought, therefore, what you shall eat, or what you shall put on; for your heavenly Father knows that you have need of these things. But seek the kingdom of heaven, and all these things shall be added unto you. For where his treasure is, there also is the mind of a man.” And, “Do not these things to be seen of men; otherwise you have no reward from your Father which is in heaven.”
Source: First Apology (New Advent)