Part 1
Humanae Vitae is reaching 50 years old.  It was written with the intention of silencing all discussion of birth control.  In the long run that didn't work.  Married couples struggled with their conscience and the issue in their personal lives and now most American Catholics practice birth control. 

 1. the assumption was--preventing conception from taking place was as bad as abortion.  But now we know that 30-50% of conceptions don't implant in the womb and are lost--and that 1 in 10 conceptions that implant are later lost through spontaneous miscarriage.  Why is that, God?
2.  The word Contraception is imperfectly understood.  It would be better to call it pre-conceptive birth control.  
3.  A few years after Humanae Vitae an American ecologist began warning us that we were nearing the tipping point of a sustainable world population.  Since then--in 45 years--the world population has more than doubled!  (going from 3 billion to over 7 billion.)
more to follow...