Harm No. 13: Sexual Desires are Aroused
By casting evil glances the sexual desires are inflamed and incited. If a person does not attain the beauty whom he was casting evil glances at, then he will fulfill his inflamed desires by committing illicit sexual intercourse with anyone he gets a hold of, even if he/she is ugly. He inflamed his desires by looking at a beautiful face and satisfies this desire by having illicit sexual intercourse with someone with an ugly face. He tasted evil glances out of greed for beauty and ended up degrading himself with such an ugly person that one cannot stand to even look at her/him.
Casting evil glances is such an evil act that does not leave you until it takes you to the final stage of sin wherein a person does not even see beauty anymore. Guarding one's private parts after casting evil becomes impossible. This is why after یغضوا من ابصارھم (the command of lowering the gazes) Allah Ta'ala reveals ویحفظوا فروجھم (to guard the private parts.)
From this we see that by protecting the gaze the private parts also become protected, and if the gaze is not guarded then the private parts also do not remain secure from sin.