It's not like his crime was hidden, you read it yourself. The only thing the 19 year old and a 40 year old who touches toddlers is that they both committed a sexual crime. Even though they are on complete opposite ends of the spectrum it's still true.
The gp poster is one of a very enlightened few. There was a person in a similar situation in my area for a while. Someone found him on the registry, and he was subsequently banned from several organizations he was previously considered a valuable member of. By this time he was in his late 20s and talking to some of the shriller voices against them, I realized they didn't understand the difference between something he did at 19 and something he wouldn't do at 28. They just assumed he was after their 13 year old daughters and nothing else. Ruined a lot of his ability to participate in society.
Similarly, there are many, many employers who will dismiss any chance for job based on mere presence on the list. No questions, no mitigating circumstance. This is a punishment that is life-long, goes far beyond the crime, and honestly, leaves little recourse for the offender to become a drain on society, or turn to further crimes just to get by. (blast me for being a stupid hippy liberal if you want, but explain how we expect someone to get a job if we deny them the ability to get a job without saying "just get a job").
I'm not sure I follow. It's also possible for someone to map everyone with felonies, is this inherently wrong? Someone with marijuana possession is certainly not the same as a serial killer but the website isn't making that conclusion.
If you disagree that having sex with a 16 year old as an 18 year old should be a sexual crime then perhaps you should fight the law itself.