44 Knowing that their father would condemn their design, Lot's daughters thought it necessary to fulfill it without his knowledge. We are told that they made him drunk, so that he was unaware of what happened. His guilt therefore is not that of incest, but of drunkenness. This, too, is condemned by the eternal law, which allows meat and drink only as required by nature for the preservation of health. There is, indeed, a great difference between a drunk man and an habitual drunkard; for the drunkard is not always drunk, and a man may be drunk on one occasion without being a drunkard.
However, in the case of a righteous man, we require to account for even one instance of drunkenness. What can have made Lot consent to receive from his daughters all the cups of wine which they went on mixing for him, or perhaps giving him unmixed? Did they feign excessive grief, and did he resort to this consolation in their loneliness, and in the loss of their mother, thinking that they were drinking too, while they only pretended to drink? But this does not seem a proper method for a righteous man to take in consoling his friends when in trouble.
Had the daughters learned in Sodom some vile art which enabled them to intoxicate their father with a few cups, so that in his ignorance he might sin, or rather be sinned against? But it is not likely that the Scripture would have omitted all notice of this, or that God would have allowed His servant to be thus abused without any fault of his own.
Source: Reply to Faustus the Manichaean (New Advent)