[PRL] McCarthy's "pornographic programming"?

[Doug Orleans]

I've been reading John McCarthy's "History of LISP" pages.
http://www-formal.stanford.edu/jmc/history/lisp.html

This paragraph is amusing/sad:

   The unexpected appearance of an interpreter tended to freeze the
   form of the language, and some of the decisions made rather
   lightheartedly for the ``Recursive functions ...'' paper later
   proved unfortunate. These included the COND notation for
   conditional expressions which leads to an unnecessary depth of
   parentheses, and the use of the number zero to denote the empty
   list NIL and the truth value false. Besides encouraging
   pornographic programming, giving a special interpretation to the
   address 0 has caused difficulties in all subsequent
   implementations.

My question is, what exactly does he mean by "pornographic
programming"?  Is this a recognized term for some particular bad
practice?  Or was he making some sort of pun that I don't get?

--Doug