I think that if you're 20 and you had sex with a 17 year old, or if you took a piss in some bushes near a playground, you're innocent. Your "crime" is being dumb, and you shouldn't be punished at all, at least any more than a severe reprimand. And you certainly shouldn't have some scarlet letter hanging over your head your whole life.
At the same time, if you're 28 and you actively sought to have sex with an 8 year old, you are dealing with someone with a deeply psychologically rooted paraphilia that is not going to go away upon the completion of the punishment for your crime. You really should go on a list, or be an outcast, because you are a high risk of re-offending.
The problem is with people say "you go on a list" and the rest of us think about the adult who had sex with the 17 year old.
Or the problem is with people who say "you should have your record cleared and forgotten" and the rest of us think about the adult who had sex with the 8 year old.
Before anyone says anything, you have to define your terminology well, or it will be taken the wrong way. We start by qualifying underage sex crimes in terms of legality such that sex with the 17 year old is VERY different than sex with the 8 year old.