Problem with mandatory sentences is that not all cases are the same and they do not allow for circumstances. I know a fine 35 year old man convicted for aggravated sexual assault when he was 17. Aggravated because there was more than 3 years difference in age. He was 17, pretty slow and developmentally delayed and going on 13 with poor parents, while she was 14, very bright, living in the fast lane going on 19, both in the same school. They started to have sex but she changed her mind and he stopped. Ten years prison. He was bragging about seeing the girl without clothes and some teachers reported the incident. Both told the same story, no conflicts, no arguments, no problems. The DA saw the chance to hit a home run while the court appointed attorney just took the cash and ran. (actually just sat there) He served the 10 hard time but is now marked forever as a pedaphile. He is not, never was, and never will be. He was an easy mark. No excuses from him.
But I think cases like the one you cited are relatively rare. Far more likely, is a serial molester, with dozens or hundreds of victims. And after such a person serves some time, is released, they repeat the offense.
In the case you cite, one innocent person suffered.
When a molester is released and repeats, more than one innocent person suffers.
Releasing molesters means higher number of innocent people suffering. I support mandatory sentences, and I feel that the low number of innocent people suffering is acceptable. It beats the heck out of the alternative.