Nuns and priests are emotional adolescents.
Because of the unnatural lives they lead, coupled with undeserved adoration and indulgence, it let's the mentally unstable among them blur the notion from being God's representative on earth, to being God on earth, omnipotent and indulged well beyond the bounds of sanity and reason.
Add in the arrogance and secrecy, and these situations are what grow and flourish.
We should not be surprised that such horrors go on, only that it not more accepted and commonplace than it already is.