Answer:
The answer to the question: What does the underlined sentence mean, would be, precisely, that Clayton, when confronted by the dog, and given the dog´s stance, had to refrain himself from assuming a protective stance towards his privates, which meant covering them up.
So the sentence literally is telling you that Clayton forced himself not to protect his privates, even though it was an instinctual action, given the way that the dog was looking at him, and acting.
The clue about this being the correct interpretation comes from the sentences written above that one, describing how Clayton sees the dog reacting to him. And thereafter, in the sentences after that one, you read that Clayton demanded that the individual place a leash on the dog, even though the dog was not moving in any way, and the individual assured Clayton that the dog would not move unless he was ordered to. Both these clues allow you to know the meaning of the sentence you underlined, as well as the sentence itself.