I'm old enough, and except for places like 4chan, and ads on torrent sites (before adblocks were a thing), I pretty much never saw porn online by accident. The closes to porn were censored boobs on social networks when someone shared some daily mail article or other mainstream media stuff.
On the other hand... There is "WikiHitler", a game where people click on a "random article" on wikipedia and try to reach the "Adolf Hitler" page in the least amount of clicks... so yeah, technically, on wikipedia, you're always a few clicks away from Hitler too, but not by accident.
These are all versions of Six Degrees of Kevin Bacon [1] (or the Erdos variant if you like) and thus no surprise at all. And these includes this "clicks needed to get to porn" variant.
On the other hand... There is "WikiHitler", a game where people click on a "random article" on wikipedia and try to reach the "Adolf Hitler" page in the least amount of clicks... so yeah, technically, on wikipedia, you're always a few clicks away from Hitler too, but not by accident.