I agree with part of what you are saying....parents should have to pass a test before being allowed to have children. There are a lot of parents out there that don't know what they are doing and buy a kid a video game because they ask for it...they don't do any research into what the game entails, they just want to make their kid happy.
You can say what you want about desensitizing, but it does happen. Whether it happens because parents don't pay attention to their children or whether it happens simply because children play these horrific video games....it happens. I remember a couple well to do families in a suburban city in Colorado that had children that played a lot of video games (and by today's standards, they were MILD, when you consider what is out there today) and re-enacted them in their high school....this incident put Columbine on the map. The point is that these children are seeing violent acts (murder, assault, armed robbery, use of weapons, prostitution, etc...) in a video game, not all children will consider it as a possibility, but the fact that SOME children will should be enough to prevent them from getting into the hands of ANY children. I know that there are video game ratings out there, but they are a complete joke. I sincerely hope, for your sake, that your kids REALLY are "doing fine"....but everyone is doing fine, until they aren't!
All it takes is one time...one time that someone upsets them and they think back to the video games that they were playing and think that it would be a way to solve a problem. I am willing to bet that your sons have disobeyed you before (not cleaned their room when you told them, came home later than you told them, lied about something, etc...)....why do you think that you would be the first thing that they would think of when they are faced with a confrontation with another person??? To say that this couldn't happen to your kids is ignorant...I hope that it doesn't, but no matter how well you prepare your kids, they are human beings.....and human beings have minds of their own that are influenced by the things that they do every day...you are only part of that (you hope that you are the biggest part of that, but you never know if you are REALLY).