How do people that can't perceive 3D view a stereoscopic 3D image or movie?
People who are unable to perceive stereoscopic depth typically only perceive information from the dominant eye.

Failure to perceive stereoscopic depth is usually caused by conditions where the eyes do not converge properly (e.g., strabismus). If left untreated, it is not possible to develop the necessary correspondance between the two signals from either eye, and one eye will dominate over the other. The result is that the majority of visual cortical neurons will respond to stimuli presented to the dominant eye, whereas there will be far fewer binocular neurons (i.e., neurons that respond to stimuli from either eye) and monocular neurons that respond to stimuli from the weaker eye.