The one who casts evil glances is cursed by Nabi ﷺ. Nabi ﷺ says in a Hadith mentioned in Mishkaat Shareef:
لعن اللہ الناظر والمنظور الیھ
"May the curse of Allah be upon the one who casts evil glances and upon the one who presents him/herself to be looked at." (Mishkaat; Pg. 270)
If casting evil glances was a minor sin, then Nabi ﷺ, being a mercy to mankind would not have cursed its perpetrator. The curse of Nabi ﷺ is clear proof that this is a very despicable crime. The meaning of curse is to become distant from the mercy of Allah. Imam Raghib Isfahani mentions in Mufradaatul Qur'aan that the definition of لعنة (curse) is:
البعد عن الرحمة
"To become distant from mercy."
Thus, he who has become distant from the mercy of Allah Ta'ala, cannot gain salvation from the evils of his Nafs. Only he who is under the shade of Allah's mercy can attain salvation from the evils of the Nafs. Allah Ta’ala says:
ان النفس لامارة بالسوء
The Nafs abundantly commands evil.”
(Sura Yousuf; Ayat 53, Juz 13)
So how is it possible to be saved from the evils of Nafs? Allah Ta’ala further says:
الا مار حم ربی
“Expect by the mercy of my Lord.”
From this, we understand that the only way to be saved from the evils of the Nafs is to come under the shade of Allah’s mercy. Allah Ta’ala who is the Creator of this Nafs Himself has proclaimed that those who are in His mercy will be saved. Therefore, the one who has come under the shade of الا مارحم ربی his Nafs did not remain امارة بالسوء (excessively commanding evil) rather it became امارة بالخیر (abundantly commanding good). This is why after the command of guarding the eyes, یغضوا من ابصارھم , Allah Ta’ala mentions, ویحفظوا فروجھم “Guard the private parts.” Because through the blessings of fulfilling the divine command of Allah Ta’ala and saving oneself from the curse of Nabi ﷺ one comes under the shade of Allah’s mercy. Now his private parts will also be safeguarded. Form this we conclude that the reward of guarding the eyes is that the private parts become safeguarded. On the contrary, if a person does not guard his eyes then he cannot guard his private parts either and curses which descends upon him is a separate punishment on its own.
More Reading: The 14 Harms Of Casting Evil Glances