Let me put it this way - you can go on google right now, type in "two teenagers kill man with screwdriver", and very quickly(on dailymotion) find an EXTREMELY graphic video of two guys killing a person with a screwdriver, picking out his eyeball and playing with it, while he's still alive etc etc. It's absolutely sickening and it shows a horrendous crime being committed. Yet watching and/or downloading such a video is not a crime in itself. If police ever found such a video on your drive they would probably go "wtf dude" but nothing would happen to you. How is that different from the video you described? Because someone might wank to it? What if someone gets off on the one I described? Is it a crime before or after you get aroused?
Instead, like I said - simply having a video/picture/text/drawing of literally anything shouldn't be a crime in itself.