A doctor had just finished a marathon sex session with one of his patients. He was resting afterwards and was feeling a bit guilty because he thought it wasn't really ethical to screw one of his patients.
However, a little voice in his head said "Lots of other doctors have sex with their patients so its not like you're the first...". This made the doctor feel a little bit better until still another voice in his head said, "... but they probably weren't veterinarians".