As I understand it, one of the primary purposes of sex offender registries is to get information out about who is on them. I believe some people are forced to knock on doors in their neighbourhood and disclose it. In other situations they would just be getting ahead of the story.

They wouldn't. But if they were sex offenders, they could claim that their offense was simply sending a nude when they were 16. While their real offense may have been rape.

I don't actually know what the law is in regard to sex offense. I'm simply explaining what I understood from the previous comment.