You all know this thread was inevitable. List on, my friends.
in the microwave
…in a naga jolokia pepper?
(Note: I don’t personally have a penis, but still wouldn’t want to get within 5 feet of one of those)
…In a pencil sharpener…
…in charge of a large corporation…
…in close proximity to a pack of hungry dogs.
in a blender.
In a toaster
…in a fight with a Sicilian when death is on the line.
Actually, I’ve spent too much time hanging around a board where medical types hang out.
The way I’d have begun this list would be:
A List of Things to Never Put In a Penis
rusty bailing wire
tygon tubing
small caliber artillery shells
anything currently alive
batteries
In a Hitler costume at a barmitzvah
in a land war in Asia?
… in a sitcom
Though I’d probably watch an episode or two of Two Guys, A Girl and a Penis, The Facts of Penis, or Penis Brewster.
…in office.
near an angry spouse/partner with sharp teeth…
new to the boards, first post. and what a topic to start… 
… or shotput a penis.
Confirming my theory that there is at least one song about everything, Penis Envy, by Uncle Bonsai. Sample lyrics…
If I had a penis
I’d take it to parties
Stretch it and stroke it
And shove it at smarties
I’d take it to pet shows
And teach it to stay
I’d stuff it in turkeys
On Thanksgiving Day
…in charge of foreign policy.
…where your mother could find it.
One of those Chinese finger trap things.
Robot Arm’s post reminds me of the King Missile song “Detachable Penis”…