"Rape fantasies" are like "square circles": By definition impossible. The main point of "rape" is the non-consensuality of it, but as soon as someone dreams of being taken by force it no longer is non-consensual, since it is wanted by the "raped" person, and hence it no longer is rape. What's more, using the word "rape" in this context is a mockery of women who really have been raped. Pfui!
The correct term for what these people dream of is "violent sex", but that has nothing whatever to do with rape.