I'll start by saying I have no sympathy for Johnson and I am not defending his actions at all. He knew what he was doing and he was deservedly punished for it.
But looking at the evidence and similar cases, it's clear that he was made an example of (and rightly so maybe!). He fingered a 15 year old that (by all accounts) looked a lot older. A psychologist at the trial concluded he was not sexually attracted to children. He is obviously an idiot and a scumbag for what he did to his girlfriend, but I wouldn't say he was an immediate danger to children. He got 6 years in prison where similar cases have got very little in terms of a custodial sentence.
Meanwhile, on 24 hours in police custody, a guy who was found with hundreds of videos and images of children as young as 3 being raped is given a suspended 12 month sentence.
Out of the two, which one is more dangerous?
I know which one I would want locked up indefinitely!