And whereas Celsus ought to have recognised the love of truth displayed by the writers of sacred Scripture, who have not concealed even what is to their discredit, and thus been led to accept the other and more marvellous accounts as true, he has done the reverse, and has characterized the story of Lot and his daughters (without examining either its literal or its figurative meaning) as “worse than the crimes of Thyestes.” The figurative signification of that passage of history it is not necessary at present to explain, nor what is meant by Sodom, and by the words of the angels to him who was escaping thence, when they said: “Look not behind you, neither stay in all the surrounding district; escape to the mountain, lest you be consumed;” nor what is intended by Lot and his wife, who became a pillar of salt because she turned back; nor by his daughters intoxicating their father, that they might become mothers by him.
But let us in a few words soften down the repulsive features of the history. The nature of actions— good, bad, and indifferent— has been investigated by the Greeks; and the more successful of such investigators lay down the principle that intention alone gives to actions the character of good or bad, and that all things which are done without a purpose are, strictly speaking, indifferent; that when the intention is directed to a becoming end, it is praiseworthy; when the reverse, it is censurable.
They have said, accordingly, in the section relating to “things indifferent,” that, strictly speaking, for a man to have sexual intercourse with his daughters is a thing indifferent, although such a thing ought not to take place in established communities. And for the sake of hypothesis, in order to show that such an act belongs to the class of things indifferent, they have assumed the case of a wise man being left with an only daughter, the entire human race besides having perished; and they put the question whether the father can fitly have intercourse with his daughter, in order, agreeably to the supposition, to prevent the extermination of mankind.
Is this to be accounted sound reasoning among the Greeks, and to be commended by the influential sect of the Stoics; but when young maidens, who had heard of the burning of the world, though without comprehending (its full meaning), saw fire devastating their city and country, and supposing that the only means left of rekindling the flame of human life lay in their father and themselves, should, on such a supposition, conceive the desire that the world should continue, shall their conduct be deemed worse than that of the wise man who, according to the hypothesis of the Stoics, acts becomingly in having intercourse with his daughter in the case already supposed, of all men having been destroyed?
I am not unaware, however, that some have taken offense at the desire of Lot's daughters, and have regarded their conduct as very wicked; and have said that two accursed nations— Moab and Ammon— have sprung from that unhallowed intercourse. And yet truly sacred Scripture is nowhere found distinctly approving of their conduct as good, nor yet passing sentence upon it as blameworthy. Nevertheless, whatever be the real state of the case, it admits not only of a figurative meaning, but also of being defended on its own merits.
Source: Against Celsus (New Advent)