Ah, the specter of thoughtcrime rises again.
I don't believe that people should be considered criminals for what they think, no matter how messed up it is. People are criminals for what they do, or at the very least what they make a serious attempt to do. You don't have to actually kill someone to be arrested, simply trying is enough even if you fail. Thinking "I'd really love to catch that guy in a dark alley" is not a crime.
The same should be extended to "sexual" crimes, although it seems like a lot of cultures are so uptight about sexuality that they're willing to criminalise any non-"normal" sexual behaviour.