Just that: is viewing or downloading pornographic images or videos on your own home computer legal? Is is safe – that is, safe from The Authorities, whoever they might be – to view porn online? By porn, I mean adult with adult, nothing with children (eeww!)
Short answer, yes. In the United States it is.
It is legal for adults to view it.
Define “porn”
Most countries have some definition of “porn” that is illegal to possess, and watching usually qualifies as possession. Where are you?
You can’t define it. You can only know it when you see it.
Which you can only do by downloading and viewing it.
I throw myself on the mercy of the court.
The courts and prisons could never keep up with the arrests.
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Has anyone, anywhere in a first world country, been arrested for downloading and viewing porn that does not involve children? If so, were they convicted?
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Are there any indications, for example law review articles, that would indicate that there are jurisdictions where downloading porn is technically illegal but unenforced?
Absolutely nobody can actually answer this without knowing where you live.
Not that you’ll necessarily get an entirely correct answer anyway, but knowing where you are is a necessary precondition on getting a correct answer.
I THINK the OP is trying to suss out how ubiquitous internet porn runs afoul of “community standards” and if the community in question can do anything about it. There ARE still cities/towns/counties that don’t allow the sale of that sort of thing, AFAIK. Does that only count for hard-copy (snicker) or can the local authorities actually request internet logs to see if you’ve visited hotgrannydonkeypunching.com?
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Several people have been arrested and convicted in Australia for possessing cartoon pornography of Simpsons and Power Puff Girls characters. Does that count? cite.
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In Australia, the legal definition of child pornography includes anything that appears to depict someone under the age of 18. Porn including a 30-year-old in a school uniform is probably illegal. But this is not widely known and rarely enforced.
What if someone only views it as streaming video or pictures on a web page, without downloading them? Is that “possession”? They could claim it was an accidental viewing but of course web logs could belie that and prove repeated traffic, but it seems that the Internet makes a lot of these “possession” laws pretty gray.
As for fan-fic drawings fictional Simpsons characters being “underage porn”, well, one could also point out that the Simpsons have been on the air for over 20 years so even Maggie has been “legal” for some time 
I’ve heard of people who make BDSM porn being arrested and their business trashed because some city on the other side of the country decided to go after them with “community standards” laws. It seems logical that if you can go after the producers, you can go after the consumers.
Yeah that’s more than a little silly. If I draw a character in a sexually explicit way that looks like a child but I claim that the character itself is several thousand years old (I’m looking at you Leharl from Disgaea), is that still child pornography?
What if I claim that the characters aren’t even supposed to be human? What fucking human being looks anything like someone on the Simpsons or the Powerpuff Girls?
Seriously, those are some messed up convictions.
What about this pornographic video game, Custer’s Last Stand. Neither of the characters have any pubic hair. Maybe he’s raping an-underaged woman? How would we know!?
NSFW: http://upload.wikimedia.org/wikipedia/en/a/ae/S_CustersRevenge_1.png
Back around 1990 or so there was a satellite (big-dish, not DirecTV) hardcore porn channel called American Exxxstasy which was available (by subscription) throughout the northwestern hemisphere (i.e. obviously everywhere its geostationary satellite covered). Some ambitious DA in a little po-dunk county in the American South (Georgia, Alabama maybe?) actually got an injunction which made the whole network go dark for several months, just because people were capable of subscribing & receiving porn in his county!
Fortunately the First Amendment prevailed (at least, that’s what my ‘friend’ said…) 
Need answer fast?
I recall hearing complaints a few years ago about how that law made pictures of small breasted women illegal.