The point is that different people are effected in different ways. In some people, dormant instability can be triggered or even created by external stimuli. Obviously it doesn't happen to everyone. However, it does happen. Like the two-year-old kid in England, whose halfwit parents thought it was really cute that he spent about six hours a day playing Goldeneye 64. One of his first words was "kill" or "die". Then there's the seven-year-old in South-East Asia, who's receiving psychotherapy for pornography addiction as sexual deviance as a result of having viewed dodgy films between the ages of two and three.
Many people aren't significantly affected by a limited exposure to inappropriate material, but others are. It depends on what they're seeing - the severity of the content, the context of what's actually happening - and on their own unique psychological make-up. The odd action movie never hurt most people, but I'm sure that daily doses of Baise Moi or :bou::bou::bou::bou::bou::bou::bou: wouldn't be quite so good for the large majority of kiddies.