Sure, some are more serious because some forms of mental illness make it so the afflicted have serious trouble doing normal things in normal society.
You can have schizophrenia that is well controlled and work a job - or be on disability, for example.
You can have thoughts and not act on them, though. If someone realizes that their attraction is to children, yet understands that it is something that hurts them and therefore, doesn't act on it and doesn't view child porn because people are hurt in it... I'm not entirely sure they are doing anything wrong. Not only that, but I don't think we help this category of folks either.
It isn't that I agree at all, but not everyone with ill thoughts act on them. I'm not entirely convinced that fake porn is going to hurt anything. Sure, I find it absolutely disgusting, but no one asked me.
If you are going to dig into this, though, I fully suggest you go off on "barely legal teen" porn, a good amount of manga, and every image that makes young folks into sexual beings, though you are going to have issues around puberty since folks are becoming sexual beings themselves and it is generally good to have trial periods on things like clothing and makeup and hairstyles.
The people who view it would like to think that. But one can certainly make the case that it's not mentally healthy, and as such is indeed symptomatic of mental illness.
The DSM is a bit vague and outdated. Until fairly recently, a number of conditions which were similar but different were lumped in as one thing in research and the media.
Some people have an OCD which makes them obsess over children but they're not interested in them per say.
Some people insert themselves as children. This could happen due to abuse and neglect in their childhood or other factors. Some people are into certain aesthetics and media like cartoons and are not so interested in actual children.
There are two proper types. Exclusive and non-exclusive.
One is the exclusive who are alien in thinking. Instead of thinking of adults as romantic or sexual partners, they're wired to think of children. Without adequate socialization, they may even think of it as harmless to do. Education is vital.
They will be very lonely and depressed throughout their lives as it is impossible for anyone to allow them to do such a thing. I can't imagine someone going their entire life without ever fantasizing or looking at some sort of porn. Even as an escape. It isn't healthy not to do so either. It would only serve to make them more frustrated and to adopt an antagonistic relationship with society.
The non-exclusive aren't just attracted to children.