Lowering one’s gaze is a means to protecting the private parts and not restraining one gaze is a means to corrupting the private parts. Therefore, if you want to protect your private parts from lewdness, lower your gaze. If you leave your eyesight roam about freely – looking at what Allaah has made unlawful- then indeed it will lead you to lewdness.

[Source: It’haaf At-Tullaab Bi-Sharhi Mandhommah Al’Aadaab’ page 87. Slightly paraphrased and abridged]