One-Eyed Monster (2008)

Ten people travel into the mountains of Northern California to shoot a porn movie. During a scene break early in filming Ron Jeremy gets hit by a shooting star and in the following scene his penis, now possessed by a sex-hungry alien, detaches itself, leaving Ron for dead. The remaining cast and crew theorize about the murdering penis' vulnerability after climax, 'when it is limp, and at its weakest'. They hatch a plan to lure the penis into a neuro-tactile simulator and then circumcise it with an ax during its 'refractory period'.