The practitioners of bhakti or devotional service to Krsna, especially those that are initiated, vow to follow four regulative principles. These are no intoxication, no illicit sex, no meat eating and no gambling. However, watching TV and movies always includes seeing others engaged in intoxication, graphic sex life, violence, and gambling. Therefore in a sense watching it means we are breaking our regulative principles of freedom.
What reason do we have that we should go out of our way to observe and watch these activities of Kali-yuga. Already it's difficult enough to stay away from them, being bombarded and having our consciousness challenged by them when simply going about our daily activities? And why would I want to bring these things into my own home, to give ideas to my children?
The effect of breaking the regulative principles is that we commit sin. By committing sin, we complicate our spiritual lives, entangle ourselves in material existence, and suffer from the results of that sin. Anything that you see makes an impression in the heart. So the rampant violence, sex, intoxication, bad language, immoral behavior, and fairytale worlds like Spiderman jumping off buildings and flying through the air, these make an impression in the heart. When they do, they become a source of meditation, not only as a reflection of the mind, but something that creates seeds of material desires in the heart and which will also water existing seeds of desire in the heart.
By committing sin there's an effect in the mind, these sinful thoughts linger in the heart. They come up like bubbles from the mud and become the source of other sinful desires, which then become the result of further sinful acts. Ultimately what devotees see on TV and in movies, they'll end up doing. If they see infidelity, they'll end up doing it. If they see violence or other activities, they'll do that. When they become bewildered into thinking materialism, egoism and pride in the material body are the ultimate goal, they become more body conscious and egotistical. All these things strangle the creeper of devotional service and make one's devotional life more or less null and void.
The solution is the plug-stoppers of acquiring a taste for reading Bhagavad-gita, Srimad Bhagavatam and chanting Hare Krishna. By regularly doing these one will naturally develop a taste for doing more. This in turn will give the spiritual strength needed to resist the social pressures from those that have no taste and wish to be sinful. A devotee will then be able to continue their spiritual journey with a clear conscience.