Another post says that Google prefers own Google services results over other organic ones, in the case of movies for instance:
http://seoblackhat.com/2006/11/21/google-serps/

I think it's a natural effect of algorithm tweaks favoring "trust", and not specifically anti-porn.

Searching

[porn +the]

or

[porn +my]

seems clean.

[porn +a] has an ex-ex-ex site as #1, but is otherwise fine.

However

[porn free]

is very different

Yes true: search "paradis p oker" see the message from Google in the bottom.
   In response to a legal request submitted to Google, we have removed 1 result(s) from this page. If you wish, you may read more about the request at ChillingEffects.org.

http://www.chillingeffects.org/notice.cgi?sID=1161

Also, it's true that most porn sites aren't going to actually use the word "porn." Words like sex, pix, nude, naked, teenage, etc. are probably more realistic search terms for this sort of test.