Rule 34: There is porn of it - no exceptions. Supposedly originating and popularized by (who else?) the 4chan imageboards, this was the first universally agreed-upon Rule Of The Internet. It is so well founded and documented with irrefutable proof that even those with only a cursory awareness of the Internet are aware of this rule (even if they don't know it has a name).
While the original architects of the Internet had grandiose goals of research and data sharing, the second it fell into the hands of Joe Everyman, it became a tool for one thing and one thing only: pornography! Now, it's not that everyone online is just looking for pornography; it's just that it's very very easy to come across. Even if you're not looking for it! Don't believe us? Do a Google image search (filters off) of, well, pretty much anything.
Got a favourite fictional character you'd like to see in a compromising position? The Internet's got ya covered!
Enjoy watching two same-sex individuals making out? The Internet's got ya covered!
Have a specific, even unorthodox, fetish for some kind of inanimate object? Make a wild guess at our answer.
The thing is, we humans are a social species. If given enough time, any individual will invariably come to think of that favourite of favourite pastimes. It's only natural that some of it will end up online as well.
The key reason the scope of it is so wide and bizarre lies in what some have come to call Rule 36: "If you've thought of it, then there's somebody out there with a fetish for it." Lesbians. Pregnant women. Pregnant men. Gingers. Giants. Cannibalism. Centaurs. Demons. Pagans. Rape machines. Armadillos. Snakes. Balloon animals. Letters of the alphabet (just trust me on that one).
There's also Rule 35, basically a guarantee that Rule 34 will remain true: "If there is no porn of it, it will be made", it is really just a matter of time before someone out there, perhaps motivated by its absence, draws it or writes it and puts it on the Internet. If you mention the inability to find porn of something, someone will make it purposefully just to prove you wrong.
But don't get your skivvies in a twist! It's not that we're all perverts; it's as the infamous musical number from Avenue Q tells us: "The Internet Is For Porn". Oh, okay, it's not just for porn. It's for advertising too. Frequently both at once.
See also Fetish Fuel and Power Perversion Potential if you really wanna blow your mind. You may wish to keep a bottle of Brain Bleach handy. Or not. Basically, if you're on the internet long enough and surf carelessly, you'll see porn of Anything That Moves and some things that don't. See also Rule 63, which gets mixed up with this.
It also doesn't help that for some people, whenever you leave them to their own devices... they seem to deviate towards sex and perversity. Let anyone have any degree of control, and they'll inevitably use it to make the characters naked, anatomically correct, insert sex comments, etc... While not a part of Rule 34, it is popular belief that Furries are particuarly dedicated towards supporting Rule 34.
For a list of creator reactions to the phenomenon that are definitely not examples, see here.