5 But they whose souls are blinded by dishonourable lust do not regard even nature, which from old time distinguished the names of the family. For under what relationship will those who contract these unions name their sons? Will they call them brothers or cousins of one another? For, on account of the confusion, both names will apply. O man, do not make the aunt the little one's stepmother; do not arm with implacable jealousy her who ought to cherish them with a mother's love. It is only stepmothers who extend their hatred even beyond death; other enemies make a truce with the dead; stepmothers begin their hatred after death. The sum of what I say is this. If any one wants to contract a lawful marriage, the whole world is open to him: if he is only impelled by lust, let him be the more restricted, “that he may know how to possess his vessel in sanctification and honour, not in the lust of concupiscence.” I should like to say more, but the limits of my letter leave me no further room. I pray that my exhortation may prove stronger than lust, or at least that this pollution may not be found in my own province. Where it has been ventured on there let it abide.
Source: Letters (New Advent)