31 And the firstborn said unto the younger, Our father is old, and there is not a man in the earth to come in unto us after the manner of all the earth: 33 And they made their father drink wine that night: and the firstborn went in, and lay with her father; and he perceived not when she lay down, nor when she arose. 34 And it came to pass on the morrow, that the firstborn said unto the younger, Behold, I lay yesternight with my father: let us make him drink wine this night also; and go thou in, and lie with him, that we may preserve seed of our father. 35 And they made their father drink wine that night also: and the younger arose, and lay with him; and he perceived not when she lay down, nor when she arose. 36 Thus were both the daughters of Lot with child by their father. 37 And the firstborn bare a son, and called his name Moab: the same is the father of the Moabites unto this day. 38 And the younger, she also bare a son, and called his name Ben-ammi: the same is the father of the children of Ammon unto this day.
In an ironic turn of events, the two daughters that Lot had tried to offer to the mob of rapists in Sodom ended up raping their father instead. Not due to the same violent lust that had burned in the heart of Sodom, but due to a fear that their family line would end unless they did something drastic.
Which is an example of how faithlessness can be a precursor to sin. When one is set upon achieving some goal, but cannot see any moral way of accomplishing it, and does not have faith that God will provide, then one is at risk of rationalizing immoral methods to achieve their wish instead. These daughters show the same failing as their father, who did not trust in God and stand for principle against the horde Sodom. Both Lot and his daughters felt it was entirely on them and their limited mortal power to solve the situation, and all three of them came up with a morally broken answer.
And initially it may have seemed that the daughters of Lot achieved their aim. They preserved their family line for many generations after all, but the nations that came of them would be corrupt and evil. The Moabites and the Ammonites were two of the heathen nations that the Israelites had to purge out of the land when they were led back from captivity in Egypt.