One, you have the not-so-uncommon addiction to porn. Because of the wide and free availability of porn content today, it's very easy for a person that's susceptible to addiction to fall into a vicious hole.
Secondly, it can lead to spreading aggressive, potentially dangerous fetishes. I might be wrong about this, but I think a lot of the dangerous fetishes wouldn't necessarily exist (or at least wouldn't be so popular) if it wasn't for them being so readily available on the internet.
Sex workers would be an entire discussion topic on its own - but I think it's safe to say that the information available on the dark net crosses all lines - from legal to human rights.
I'm not suggesting we should shun it or disallow it completely - but I think celebrating would also be an extreme that's not worth the consequences.