It's gross that you're attempting to oversimplify this and ignore relevant details.
a) He likely didn't know it was a video of an alleged sex assault. No one else viewing it, including the police and the school, seems to have come to the conclusion it was a video of sex assault.
b) Even the law differentiates between the punishments for child porn depending on how old you are in many states (including Nebraska, btw). I'm sorry if this rationalization is gross to you, but an 18 year old having a video of his ex girlfriend when she was 15 that he received when he was 16 is not the same as a 60 year old having child porn videos of 8 year olds. But you're lumping them together by saying it's gross to add nuance to the discussion and that "child porn sex assault" should be the end of it.