I am personally of the opinion that in hentai and other cartoon sex, there should be few, if any, boundaries. By making certain practices taboo or banning them from the fantasy sphere, we are taking away a way of fantasy expression without there being any conclusive proof that the depiction of said practices induces any actual enactment of said practices. From a personal point of view, I, from time to time, rather enjoy the roughness of some hentai scenes that sail close to rape scenes but I would never in a million years consider committing rape.
I think society is starting to lean towards restricting freedoms to protect the weak from themselves. I don't think this is a good thing as in my experience tabooing a subject or practice does not make it disappear. I have always been for exposure of unaccepted practices. Exposing does not equate to trivialising.
In this way, I think there might be something to say for the "let them get it out of their system"-argument; by letting people get it out of their system by seeing the unaccepted deed committed in non-realistic parametres, they may not feel the need to enact the deed in actual life.
As far as I am aware, there is little or no proof that the human mind actually functions in such a way, but I can imagine it would. In a very cynical way, I wonder if it really is that good to protect the many from the few by taking everyone's liberties away. Recently there was the discussion of whether or not to ban the videogame Grand Theft Auto 4 as it might induce violence (a kid in Thailand, I believe, stabbed a taxi driver to see "if it really was that easy"), but I would argue that even in such situations the few should not rule society's freedoms.
I would, however, be more hestitant to support fake rape scenes involving actual people/actors. Hypocrisy? Perhaps, but I can't be expected to be consistent....