These are people (both priests and nuns) who are taking advantage of people (children, teenagers, young and old women and men) who are vulnerable and trust the priests and nuns. In my mom's church, one of the priests raped a 22 year old woman and in a nearby diocese one of the nuns was causing several of the older men and women a great deal of pain by telling them that they were 'weak' for wanting pain medications! Because they trusted her and believed her when she said 'god was ashamed of them' they spent months in pain and one of them died in agony.

The idea behind letting nuns and priests marry is that by allowing them outside relationships they will have proper pressure valves for the stress they are under. Nuns and priests have a great deal of authority and power and yes, control over their parishoners. That sort of control and power, when it has nothing outside to help keep it in perspective, leads to abuse. It happens in normal relationships, of course and there is no guarantee that some priests and nuns will continue to abuse the people who trust them, but it is a step towards a more healthy church.