Musar על שבת 106:13

Every man is in a position to know for himself, just what kind of a fence he should build, in accordance with his own requirement. But the best of all fences is that he should guard his eyes from longing for what does not belong to him. And if he lessens his eyeing of things that are not his, he will be called modest, and bashful. As was said concerning the woman who had one arm amputated and yet her husband was not aware of this until the day of her death. And they said of him, "How modest is this man that he was not aware of this in his wife!" (Shabbath 53b). And if he guards his eyes, his heart will also be well-guarded. And since his eyes and his heart are well-guarded, he will be well-guarded through and through.