You'll never get as accurate a transaltion as when you use a native, human speaker I'm afraid - things like colloquillisms never translate very well (for example, the french version of 'its raining cats and dogs' is 'its raining buckets' - a direct translation, as the software will get you, may actually be meaningless). Have you not thought of advertising for a German speaker to go through it and translate? If you go through a site like freelancer.com you can ask for a 'German speaker to translate erotic story' and offer as little as a couple of dollars for the job completion.