How possible is it to 'set no evil thing before my eye', as the psalmist says? Certainly it is more difficult today. In king David's day, one had to search for evil, right? Whether or not that is true, today, one must search to find pure things. Why would God, knowing how difficult it would be for us Christians in the future, ask us not to look, hear, or experience anything evil? Certainly He must have had good reasons.
I contend that the first, and probably the most dangerous, reason is what I call normalizing, otherwise known as desensitizing. If I look at violence on TV, or in music, my mind and heart generally feels more comfortable with it. I'm not saying I'd go out and do such things, but I don't see them as abnormal. Bad? Maybe, but not necessarily abnormal. I see homosexuality on movies and TV for years and begin to think it is a part of normal life. I still may think it's wrong, but not abnormal. Eventually, however, what we see as normal, eventually becomes what we accept as okay. Finally, what we accept as okay, we eventually either do, or defend someone's right to do it. This slow, gradual process of mind changing (brainwashing) was used in almost every cult-nation, like Nazi Germany, Communist USSR, and so on. If you don't believe me that normalizing is dangerous, check out my paper on the Mass Media and its influences on our culture.
This normalizing effect can be seen in even more seemingly harmless actions, like dirty jokes. The bible directly says coarse joking is wrong. This is because of the normalizing effect. Many years ago, dirty jokes were not an accepted part of the American society, because we were a Christian nation. Later, only dirty people told dirty jokes. Even later, telling dirty jokes around women was wrong. Finally, women can tell dirty jokes, and now, dirty jokes are about all that is left. The normalizing effect does not end there, however. Joking about sexual encounters may eventually lead you to believe that these things are normal, wrong but normal and so on.
Another possible reason for this difficult command, is the delicacy of the spiritual closeness we desire with God. One aspect of a Christian walk, is wanting to be close to God. When you expose yourself to things such as harsh music with dirty lyrics, movies with dirty themes (not just porn), jokes with dirty punch lines, you will feel dirty. It may not seem like dirt, but it will when you get close to God. An Old Testament prophet said, when seeing God's power, "Woe is me, for I am a man of unclean lips, and I live among a people of unclean lips". When your lips, or eyes, or ears, are unclean by being exposed to this worlds offerings, you will not desire to be close to God, because he is clean, and people don't like to see how dirty they are. This loss of this desire is not only contrary to God's will, it is a sin by itself.
The last reason is a strange and foreign idea to most Americans. It is my opinion that there are two kinds of excitement in this world. There is the excitement of the world, and of God. God's excitement is made up of peace, joy, love, thankfulness, kindness and so on. The world's pleasures are more complex, louder, more colorful and quicker. They include physical thrills (sex, speed, drugs) and emotional thrills (power, danger, loud music). The things of this world that are including in 'putting evil in front of your eyes' are worldly thrilling. This type of thrill is so loud, it makes it impossible to hear God's "still, small voice" (1Ki 19:12) that whispers to us His will. I can't hear God's will for my life while watching action films, or listening to screaming music. I must be at peace, rest, quiet, to hear his voice. God wants us to be happy, content, and even excited, but only in him, not excited in this world. What would be the point in getting excited over perishable things anyway. Get excited over God's eternalness.
As you can see, there are many reasons behind God's rule for not 'putting evil in front of your eyes'. Exposing yourself to evil can normalize it for you, destroy your closeness with God and even block his voice out. I must confess I am not perfect in this vein. I watch movies with bad scenes and themes. But I also know that this is not the will of God, and I want to do only the will of God, for in His perfect wisdom, His will is perfect.