Before deciding how we should punish people you have to consider the purpose, and how punishment achieves the goal. Criminal justice serves many purposes -- removing criminals from society, punishing people for bad deeds, restoring a sense of justice, rehabilitating bad people so they will be productive in society, deterring others from committing the same crime, vindicating the victim. There are probably more.
As a society we try to stay civilized and not sink to the level of our worst criminals. Even if it's just their way of expressing disgust I don't think it's responsible for people to talk about wanting to castrate, rape, shoot, etc., people for having commited a sex crime. Revenge violence is still violence and, other than restoring a sense of balance, it doesn't help the problem.
Most likely any system needs some degree of proportionality. It's all bad, but the relatively minor crimes like being a peeping tom or flashing are not nearly as horrific as rape. For a 19-year-old to have consentual sex with a 17-year-old is usually a crime, but not nearly as bad as with young children. Someone can accidentally or stupidly come across a piece of free internet child porn made in Europe, which is a lot different than participating in the child porn trade. Finally, some crimes are likely to happen again and again, others are unlikely to be repeated. For all these reasons there is no one-size-fits-all punishment.
Most child abuse happens at home and most of the perpetrators are parents or other relatives. The abusers have usually been abused themselves as children, and are perpetrating a cycle of violence. If you add physical abuse to the picture, which is related and almost as damaging as sexual abuse, there are probably millions of adults in this country walking free who have abused children. Most are never caught, and even if we could, we can't put them all in prison for life. There just isn't enough money or room, and we would create a lot of orphans.
Punishment is only a final backstop against child molestation after everything else has failed. By the point the law steps in the child is already a victim. The long-term solution is to break the cycle by getting to the victims before they reach adulthood and do it themselves, to educate children that they can stop the abuse, and to intervene immediately in families where there is abuse. Of course, arresting and jailing abusers who are likely to do it again is a big part of the picture too.
You should be aware that those convicted of sex crimes are essentially serving a life sentence. If they have to register as sex offenders then everywhere they go the neighbors find out and they usually get chased out. In any setting they're vulnerable to blackmail. They cannot get a job either. I can understand why people support these laws but if not done right they can be unhelpful and counterprouctive. For example, some laws say registered sex offenders cannot live within 1,000 feet of a church or school. That sounds nice until you realize that the type of crime they commit does not involve churches and schools directly, but the restrictions mean they cannot live in the inner cities so they all move out to the suburbs and countryside -- where there are fewer cops, social workers, etc., to keep an eye on them.
Unfortunately there are a lot of laws like that that are motivated simply by anger about the problem, without a whole lot of thought given to actually solving it. And of course politicians are eager to jump on the bandwagon wherever they can get a vote, so they're not always interested in solving the problem either.