You don't draft penal codes thinking "Bad people will look at this and stop what they are doing now that they know their behavior is wrong, which they didn't before we told them." That's stupid on at least 17 different levels. Possibly more.
Laws exist in order to give We the People a means of dispensing justice. That's it. Rule of law. We don't like vigilantes because they cause chaos and trample on peoples' rights. So we set up carefully defined and restricted systems that grant us official permission to prosecute one another. If laws like this are never put in place, then we don't have the OPTION of prosecuting anyone. Ever.
And it is an option. Prosecutors will not pursue every single Tom, Dick, and Harry who has a drawing on their computer of a kid fellating a unicorn. So rest assured that your allegedly non-perverted self won't become some dude's bitch in prison.
As for your "harmless" statement, I find it incredible that you would paint with such broad strokes when defining what art is. Pun intended. Ask any doctor, psychiatrist, criminal profiler, cop, state advocate, teacher, or historian (to name but a few) how innocuous it can be. Art has been a profound expression of emotion for thousands of years and I'm stunned that you've apparently completely written it off.
Make no mistake, I don't hold every doodle on a napkin as having some pronounced meaning. But when the intent and volume of the imagery is quite obviously to obtain sexual gratification from the abuse of children I take that very seriously, and so do most people.
Is it less sick to dismiss it with a laugh than view it with your hand down your pants? Quite frankly I'm not sure. You either tolerate perverts or you are a pervert. What's the difference?
And how many innocent people get caught with child pornography?