Its about choice and personal freedom. Who judges that someone has a "fetish that realistically can not be played out" and if they are found to have one, should they be castrated before having been found guilty of anything?
Anyone can castrate themselves if they want to, but taking the choice for them as a form of punishment should merit a crime on behalf of the castratee.
Keep in mind that the negative externalities of the fetish are only felt if they act upon it.