Exploiting children? Absolutely should be prosecuted. Paying for CP (and thereby directly contributing, economically, to its production)? Definitely. But just downloading and viewing an image? I'm no fan of the idea of CP, nor a radical libertarian, but I just can't grok this idea.
I've heard many justifications for it. The most substantial of these seek to establish some connection between demand for this content (by non-paying/free viewers) and the economic motive to create it. I think many people in this thread--and numerous investigations--have found this link to be tenuous/nonexistent. What, are child porn producers running a charity or something? They're in it for the money. If you download pirated/free CP, they don't get paid, and they'll never get paid.
The more ridiculous explanations, frequently deployed by Jesus politicians and law enforcement lobbies, have to do with a link between consumption of CP and real-life sexual abuse/assault/molestation/rape of minors. Even if we assume, for a moment, that there is some connection and that the causality is intelligible[1], how is that compatible with the spirit of hundreds of years of Anglo-American liberal intellectual history and legal theory? Since when is it okay (or constitutional, etc.) to prosecute someone for something they might do, but haven't actually done? Is this possible in any other area of law, or is CP that special? Why's it so special?
[1] Does CP cause sexual abusers of children to go do that? Or is it something they take an interest in it because they're already sexual abusers?