Looking for story based in feudal Japan

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I may be missing details, but it includes an explorer visiting a powerful shogun/samurai. There's a rule that if he cums he will be put to death and one of the geisha's decides to give him a blowjob as he resists. I don't remember how it ended, but there was a line that really stuck with me about how he'd defeated so many men, but it was this petite woman who defeats him.