There is a theory that says that the Venus of Willendorf, for instance, was “porn”. Quite a number of sexy-geeks find this kind of thing unfailingly funny, since it is not a mainstream theory and surely needs a zany imagination to conceive.
To productize this idea to xkcd standards, several obscure references are brought in and the thought distended to strip length.
There is near-infinite discussion on the web of the mistake in the long word; you are invited to google it. Or as xkcd fans would suggest, duckduckgo it.
text/x-kcd 