For the girls, it was then end of the human race as they knew it. Their desire was to perpetuate the species, to be the month of the new race of human.
If sheep, horses, camels and other animals breed with their aunts, sisters and daughters, this was the only way the girls believed there was hope for life to continue.
When you are the only people on an island and you believe that is the limit of human life on the planet, you follow the example of adam and eve or cain and his sister.
Considering what they must have been thinking, that life was being destroyed, it was a brave if misguided attempt preserve the human race.
Lot was a good an but he would have sacrificed his virgin daughter to the mob if it had come to that. The girls were making sure they would not be the last ones, that their being saved had some purpose.
It is not like they had sperm from any other source or knew of any way to artificially inseminate themselves.
Given their limited knowledge of the world, I wonder what many of you would have done under the same circumstances
If it was a brother and sister alone on the earth? The girls had no brother, they had only their father.
This was a time when Abraham and his wife Sarah were brother and sister. A time long before the ten commandments. Siblings and cousins marrying was part of the norm not the exception, even fathers marrying their daughters.
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Brother-Sister and Father-Daughter Marriage in Ancient Egypt