In a way, Google already is a porn machine; it is what you make of it. But it turns out that a handful of bizarre, technically impossible search queries turn the raunchiness up like 20 notches and cause the engine to return porn results almost exclusively. The searches themselves aren't dirty in the slightest, either. It's ridiculous.
The bug first surfaced on Quora and seems to revolve around Googling strings that have contradictions, and shouldn't bring up any results. The query that tipped someone off to the bug was -4^(1/4). Turns out that is processed by Google as -4 "1 4". When you search that, Google should try to return results that have the phrase "1 4" but nothing with an 4. That is to say it should return nothing. It doesn't though. It flips out and returns a bunch of porn.
Weirdly enough, -4 "1 4" doesn't seem to work anymore, but -4 "4 1" still does. Here's what it looks like (all text, but obviously this is a little raunchy):
The porn results don't show up for every contradictory search query. It seems that they generally need to include a number and that the exclusive part of the search comes first. It's quite possible that Google's already in the process of solving the weird issue, but here are a few searches that still work at the time of this writing, if you want to see for yourself.