If by dirty thoughts you mean fantasies that, if they were real, would be horrible overall, then here's one.
Imagine if a human (man, woman, other gender, whatever) and a dog became lovers, and then, suddenly, the dog considered itself in charge. He would aggressively tell the human when they had sex and how (several times a day, however he pleased), and through bites and scratches would condition the human to never resist.
Eventually, the human wouldn't bother wearing clothes and would shower only sparingly. The human would feed the dog whatever he liked, play however he wished, and lose contact with the outside world, depending on the dog for everything emotionally.
There is something arousing about the idea of truly being a bitch in every sense, but one would have to be psychologically broken to carry it to that extent in the long term, and the dog would ultimately show fearful aggression due to the isolation and uncertainty.