"A girl three years and one day old may be given away in betrothal by her father by way of sexual intercourse"?

We do not know the social customs and practises prevailed at the time of writing this.We can not judge past from present conditions.The marriage system is not older than 2500 years.In earlier times,in the beginning of human race,population was small,sex between daughter and father,in between family members was prevalent.When father was dead son used to marry mother.We can not follow these in present civilised society.We have to stop comparing prehistoric practices with practices of our times.