Why did Lot in Genesis 19, get drunk, have sex with his daughters, and get them pregnant? I wondered what was the purpose? Couldn't he have just said no to his daughters?
A major problem with all drugs, including alcohol, is that they impair a person's judgment. Things that normally would be rejected somehow become "reasonable" to the person under the influence. "Woe to him who gives drink to his neighbor, pressing him to your bottle, even to make him drunk, That you may look on his nakedness!" (Habakkuk 2:15).
There were numerous places in this account where Lot could have and should have done things differently. He shouldn't have gone into hiding, leaving his daughters with the impression they would never be in civilization again. He shouldn't have started drinking and allow his daughters to get him drunk.
However, you make it sound as if Lot was aware of what his daughters did. "So they made their father drink wine that night. And the firstborn went in and lay with her father, and he did not know when she lay down or when she arose" (Genesis 19:33). Lot was too out of it to be aware that his daughter climbed into bed with him. Since much sexual response automatically comes in response to stimulation, it isn't beyond imagination to understand that he likely only thought he was having an erotic dream.
It wasn't until later that he figured out what his daughters had done to him.