Humanæ Vitæ is a doctrinal document of the Holy See and therefore magisterial. Catholics, along with the rest of the world may ignore it at their peril.
Catholics believe or they ought to as Catholics, that such teaching as contained in Humanæ Vitae is magisterial and therefore binding upon them. Catholics also believe that according to Christ's promise He will be with His Church until the end of time. He sent His Holy Spirit to guarantee this promise. As far as doctrine is concerned the Holy Ghost prevents the Church from teaching error, miraculously if you you like.
The abuse issue is of a different nature, it is purely administrative and legal dependent upon how individual bishops and their Curiæ tackled it; some wrongly and some correctly  the intention behind both was the good of the Church.
Popes and even Councils do not have the Holy Ghost whispering in their ear how to govern the Church, He only prevents them from contradicting the Word of God.