OK, it looks like everyone's getting the hang of this game.
Simply replace any (1) word from any title with 'penis' and the appropriate suffix (if applicable). Usually longer titles (4 or more words) work best, or instantly recognizable titles.
The main rule is if it's funny, and is grammatically correct in usage, it's good. And remember, penis can be used to replace any noun or a verb in a title (ie: 50 Ways to Leave Your Penis vs 50 Ways to Penis Your Lover vs 50 Penises to Leave Your Lover vs 50 Ways to Leave Penis Lover vs Penis Ways to Leave Your Lover - Yeah, only the first 2 are any good, but...).
Now I think that you're ready for the advanced rules:
Step One - Yell out: "Hey kids, let's play the penis game!"
Step Two - Ask all of the bouncers/rednecks/police officers/lawyers/family members/religious nuts in the vicinity to, please, put away their respective bats/shotguns/pepper spray canisters/summons/notes of shame/Jack Chick tracts. Then slowly explain that there are no actual penises used in "The Penis Game".
Step Three - Someone picks a category at random, such as 'Roger Corman Films'.
Step Four - Everyone shouts out answers such as:
The St. Valentine's Penis Massacre
The Masque of the Red Penis
Penis and the Pendulum
The Little Penis of Horrors
Penis from the Haunted Sea
The Beast with a Million Penises
It Penised the World
Attack of the Crab Penises
The Saga of the Viking Women and Their Voyage to the Penis of the Great Sea Serpent
Take a drink. Keep going until you run out of applicable answers, or the current subject gets tired.
Step Five - Your girlfriend's "artsy" friend will pick a category like 'Belle & Sebastian songs'. Try not to look too bored/annoyed while he/she belts out gems like:
"You Made Me Forget My Penis"
"My Penising Days Are Over"
"The Penis Done Wrong Again"
"Judy and the Dream of Penises"
"Waiting for the Penis to Rise"
"Le Penis de la Bourgeoisie"
Order a shot while this round is going on.
Step Six - Take another drink (it definitely makes the game better). Someone else yell out another category at random, and the game keeps going.
Step Seven - Stop once everyone passes out, or the owner of the bar threatens to call the cops.
There you go. Them's the rules (that I just made up).
Oh, and (if you can) yell out 'Sylvester Stallone movies featuring Golden Girls cast members' as a topic. Just because 'Stop! Or My Penis Will Shoot' is really fucking funny.