By the way, you did not specify whether your hypothetical 'completely willing partner' refers to to a unique partner or to an imaginary endless supply. You wish to leave ethics out of the equation here, which is a nifty idea because it makes the question more intriguing. The logic of cause and effect can not be avoided. In reality my actions have results. In fantasy and fiction I can always start from scratch.

Many of the truly violent fantasies end up with a character either permanently damaged or dead. No loss in a fantasy or story: the imaginary character was never there in the first place. The imaginary perpetrator can move on to the next imaginary victim. The plot can end or continue. There is little difference here between sexual fantasies and any fiction in which characters get injured or killed.

For example, I'm a sucker for scenes with death threats. A character who is afraid for his life and pleads for his life always makes me happy. It is the description of his fear and panic that turns me on. The outcome is optional, I don't really care if the character gets killed or is granted mercy or escapes. Depends on what fits in with the plot. This fancy would be impossible to translate into reality under the premise you specified. If I make a death threat, where's the 'willing'? And if I kill, where's the partner?

Irreversibility is an issue in fantasies about turning a prude into a plaything or an arrogant individual into an obedient slave. In fantasy I can always invent the next prude or the next arrogant bastard. In reality I might run out of candidates after a while.

On the submissive side I would assume many people fantasise about having irrevocably damaging things done to them, and for reasons of self-preservation are happy to keep them in the fantasy realm.

Having pointed out this premise problem – one of the many located on the boundary between fantasy and reality – I still think the question is an interesting one.