Well, the pun was "mentally defecative". IIRC, in the joke the guy had three conditions: first: no kisses; second: children, if any, were to be brought up in the faith; and then as the third one, he wanted to be able to pay for it on installment plan, not even understanding that it was he who was to be paid for it. But then again, it was an old joke.
Here, I thought bestiality and inSTALLments were they key to the joke.