The fetishes don't add new actions during sex, they add new Desire requests and improve the response of the person to actions corresponding to the fetish. The requests are randomly selected from the pool of possible requests. The better the opinion of the fetish the more times it is added to the pool. Many of the fetish requests can only be selected if they are not already plausibly fulfilled, meaning that they will not request cum on their body if the still have cum on them from a previous interaction.