The eye sees, and the heart desires (Rashi, Numbers 15:39).
People cannot help when an improper impulse comes to mind, but they certainly can stop themselves from harboring the thought and allowing it to dominate their thinking. Yet, sometimes one may be responsible even for the impulse itself.
While some impulses are completely spontaneous, others arise out of stimulation. If a person reads, hears, or sees things which can provoke improper thoughts and feelings, he or she is then responsible for the impulses that are the consequences of that reading, listening, or observing.
This concept is especially important in our era, when not even a semblance of a code of decency exists as to what may or may not be publicly displayed. All varieties of media exploit our basest biological drives.
Given the interpretation of the right of free speech under which such provocative displays occur, the government has no way to restrain them. However, each person has not only a right, but also an obligation to be his or her own censor. No one has to look at everything that is displayed nor hear everything that is broadcast. Those who fail to exert their own personal censorship are tacitly stimulating immoral impulses, and for that alone they are liable.