The fantasy defense is where a defendant accused of attempting a crime (enticing minors into sexual activity, for example) claims that they never intended to complete the crime. Instead they claim they were engaged in a fantasy and in the case of luring a minor, believed they were dealing with an adult.
The fantasy defense was developed by Donald B. Marks, the attorney for Patrick Naughton, the Disney executive who eventually pleaded guilty to traveling in interstate commerce with the intent to have sex with a minor, in violation of 18 U.S.C. ยง2423(b). The "fantasy defense" used in the Naughton case was novel; however, since the closely watched Naughton fantasy defense was successful, defense lawyers were expected to use it to help other paedophile clients.