Not all dolphin rape, and apparently the definition of it in animal species other than our own is different.
See the Wikipedia article on rape, which states, in part, "Sociobiological perspectives
Main article: Sociobiological theories of rape. Some argue that rape, as a reproductive strategy, is encountered in many instances in the animal kingdom (i.e: ducks, geese, and certain dolphin species). It is difficult to determine what constitutes rape among animals, as the lack of informed consent defines rape among humans. See also Non-human animal sexuality."
Note that it says . . . "as a reproductive strategy", which leads me to believe that it is not for mere sadistical pleasure, as it is for humans. And also note that it says "certain dolphin species", which means not all. Although I do not know which species do and which do not, somehow or another I don't think it's the one that's used to swim with or for therapy purposes.
The article also mentions "ducks and geese" as others that rape, but also for reproductive purposes and not for sadistic fun.
Other articles that I found on searching were written by "regular people" and not zoologists, mammaligists, or marine biologists, so I think should be discounted as not being "fact". Be careful what you read, not everything is true.
Good question, by the way.