Food for thought (Genesis 19:1-38)

In the biblical story of Sodom and Gomorrah, God sends two angels to warn Lot and his family, who are the only good people between both towns, to flee. The men of Sodom demand Lot sends out the angels, so they may rape them. Lot offers to send out his virgin daughters instead. The men refuse and demand the angels. Lot and his family flee, but his wife looks back and is turned into a pillar of salt. As they travel searching for a new home, Lot's daughters begin having sex with him in his sleep to carry on the family bloodline.

What exactly is the moral here? Offer your daughters up to be raped, and they'll sleep with you later?