Also, this was not God approved. There are many "sins" that happened before the Mosaic law went into effect where God did not punish the person. There was not as of yet a written code.
But the point is, Genesis is a history book. It isn't a theological book for the most part. Because something is recorded there, that doesn't mean that God approved of it. Lot's daughters schemed to get Lot drunk and to have sex with him. If anyone was to be punished, then it would have been them.