Yes. We can't arrest people for being pedophiles, because that isn't a crime, it's a medical condition. The legal terms for the two requirements of a crime are "Mens rea" and "Actus rea", the guilty act, and the guilty mind, without both of which, there is no crime. ...I did not mean for that to rhyme.
..Or that. Moving on!
If they haven't harmed anyone, then they've committed no more crime than someone imagining committing a murder.