Today the ‘purity laws’ refer to purification after childbirth.  They may seem outrageous, but perhaps if we try and separate them from the connotations we may have about what it meant to be ‘unclean’, they will seem less outrageous, and reveal more of the gracious character of God.  If you were ‘unclean’ you were essentially not in a fit state to take part in worship.  In this sense we could perhaps view this passage as God’s protection of the vulnerable, and his providence of time to recuperate before taking part in active worship.  The sin offering to be given afterwards then would be to cover the time of absence from worship, rather than the act of giving birth to a child.  The connotation of ‘unclean’ with ‘sin’ is also incorrect and unhelpful here.  The fact that Mary undertook the purification ceremony after the birth of Jesus (Luke 2: 22-24) should rid us of any notion that God seen the birth of a child as sinful!  There were just appropriate and inappropriate ways to approach the temple and take part in worship and they referred to physical as well as spiritual matters.  We can therefore assume that, whatever the reason, the distinction in time needed for purification between a boy and a girl, was equally gracious, and reflective of a caring loving Father looking to protect the physical and spiritual wellbeing of mother and child.  Pray that where you do not always understand Gods intentions, you would trust in His provision and grace.