Defrocking or spending the rest of their life in monasteries is not the right, just or merciful punishment.  Life time jail with no chance of parole is the appropriate punishment for these monsters.  Anything less is a joke.
We decided long ago that clerics were subject to secular law and not immune from it due to their position.  Thomas Becket is a saint, but was wrong to insist that priests could only be punished by the Church and not the king.