Although I understand the misgivings in the OP, I think it's Illegal for good reasons. It does lead to increased exploitation of Children and all those under the current Legal Age in which acting in Porn is allowed. Although the Viewer may not be directly harming anyone, their Demand for such material encourages others to Supply and thus directly harm the under aged. Exploiting the under aged in this way simply should not be allowed, thus the Viewer is culpable.
That said, there really needs to be some distinction and nuance regarding the Crime. I highly suspect that most people who have consumed Internet Porn have inadvertently seen some Under Age material. Many have likely Saved some even, but have no idea that the person(s) are <18, for they may look 18+, but be anywhere from 15ish-17ish. Should people possessing Material within that age range, but also mostly material >18 be treated the same as someone possessing pictures of Toddlers? Definitely not, IMO. In fact, I would say most of them shouldn't be prosecuted at all, just warned about it. Others who have primarily under aged Porn 14-17 are probably knowingly collecting it though and should be prosecuted, but their crime should not be equated to those who are collecting even younger aged porn.
The Law/Courts are probably aware and make these distinctions on their own though, I would hope anyway. However, when charges of making Child Porn are being made against 2 Under Aged kids taking pics of themselves of a sexual nature, I rather think the Law is being an ass. That's not something that should be encouraged, but at the same time, especially when the material was never meant to be distributed, it seems to be a violation of Free Speech.
I also suspect that some people have been charged with possession of Child Porn when they shouldn't have been. Especially in the early days of the internet. When the Police have to scour your PC all the way to recovering Deleted Files, chances are/were good that the person inadvertently came across material they simply did not want. For eg, way back before 2000, I would peruse Newsgroups for porn. At first each category would contain the titled type of porn, but eventually spammers from other genres would begin to spread their wares across many groups. At first it was rather innocuous, but eventually Child Porn, clearly Under 10, began to be spammed across numerous groups. Technically speaking, even though I was in an Adult Group, my browser History and Deleted Files of Child Porn were on my PC. Shortly after that started happening I just quit Newsgroups altogether as at the time the Law seemed to be fumbling about trying to come to grips with the Internet and Newsgroups were just becoming a spammy mess.
So anyway, tl:dr
- Intentionally seeking and viewing Child Porn needs to be Illegal
- Unintentionally viewing and possessing "Child Porn" needs to be assessed on whether the Viewer is aware of the ages involved or perhaps whether they intended for older, but acquired younger inadvertently.
- There needs to be a distinction between certain Age groups within "Child Porn"