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David Benioff is one sick bastard ... 'cause the "Game of Thrones" co-creator tells TMZ he was actually LOOKING FORWARD to shooting the Bobbitt-ing of Theon Greyjoy scene for "quite some time."

Benioff -- who runs "G.o.T." along with D.B. Weiss -- was leaving the gym in L.A. yesterday when we asked ... as a man, how difficult was it to produce that crazy brutal de-penising scene??

We also asked if the torture will ever stop -- and while Benioff wouldn't give a definitive answer, he did give us this:

"All I can say is ... we love Alfie [the actor who plays Theon] but we also enjoy torturing him."

One more fun fact -- apparently there's a casting director who gets to screen all the naked extras for the show. Ya gotta hear our photog beg David for a job as her assistant.