19 Some one may say, “Do you dare detract from wedlock, which is a state blessed by God?” I do not detract from wedlock when I set virginity before it. No one compares a bad thing with a good. Wedded women may congratulate themselves that they come next to virgins. “Be fruitful,” God says, “and multiply, and replenish the earth.” He who desires to replenish the earth may increase and multiply if he will. But the train to which you belong is not on earth, but in heaven. The command to increase and multiply first finds fulfilment after the expulsion from paradise, after the nakedness and the fig-leaves which speak of sexual passion. Let them marry and be given in marriage who eat their bread in the sweat of their brow; whose land brings forth to them thorns and thistles, and whose crops are choked with briars. My seed produces fruit a hundredfold. “All men cannot receive God's saying, but they to whom it is given.”
Some people may be eunuchs from necessity; I am one of free will. “There is a time to embrace and a time to refrain from embracing. There is a time to cast away stones, and a time to gather stones together.” Now that out of the hard stones of the Gentiles God has raised up children unto Abraham, they begin to be “holy stones rolling upon the earth.” They pass through the whirlwinds of the world, and roll on in God's chariot on rapid wheels. Let those stitch coats to themselves who have lost the coat woven from the top throughout; who delight in the cries of infants which, as soon as they see the light, lament that they are born. In paradise Eve was a virgin, and it was only after the coats of skins that she began her married life. Now paradise is your home too. Keep therefore your birthright and say: “Return unto your rest, O my soul.” To show that virginity is natural while wedlock only follows guilt, what is born of wedlock is virgin flesh, and it gives back in fruit what in root it has lost. “There shall come forth a rod out of the stem of Jesse, and a flower shall grow out of his roots.” The rod is the mother of the Lord— simple, pure, unsullied; drawing no germ of life from without but fruitful in singleness like God Himself. The flower of the rod is Christ, who says of Himself: “I am the rose of Sharon and the lily of the valleys.” In another place He is foretold to be “a stone cut out of the mountain without hands,” a figure by which the prophet signifies that He is to be born a virgin of a virgin. For the hands are here a figure of wedlock as in the passage: “His left hand is under my head and his right hand does embrace me.” It agrees, also, with this interpretation that the unclean animals are led into Noah's ark in pairs, while of the clean an uneven number is taken. Similarly, when Moses and Joshua were bidden to remove their shoes because the ground on which they stood was holy, the command had a mystical meaning. So, too, when the disciples were appointed to preach the gospel they were told to take with them neither shoe nor shoe-latchet; and when the soldiers came to cast lots for the garments of Jesus they found no boots that they could take away. For the Lord could not Himself possess what He had forbidden to His servants.
Source: Letters (New Advent)