Some people think that guarding the eyes isn't actually an obligation, but rather a good habit. But it is the Torah obligation, as it states (15:39) עֵינֵיכֶם וְאַחֲרֵי לְב ַבְכֶם אַחֲרֵי תָתוּרוּ וְלֹא, "You shall not wander after your hearts and after your eyes..."
Mishnah Berurah (75:7) writes: "All opinions agree that to look at a woman, even at her pinky, and his intention is for pleasure, it is transgressing the lav of אחרי ...תתורו לא עיניכם."
Shaarei Teshuvah (3:64) writes: "Many make the mistake, and they desecrate their holy senses... Regarding the sense of sight it states עֵינֵיכֶם וְאַחֲרֵי לְב ַבְכֶם אַחֲרֵי תָתוּרוּ וְלֹא. This means that we mustn't look at married women or other arayos, lest he falls in sin..."
Derech Pikudecha ('ו אות הדיבור חלק ה"ל ת"ל) writes: "Many people think that this is merely an idea of chassidus, and they don't refrain from looking at women. Know, my friend, that this is an issur d'Oraysa."
The Rambam (Hilchos Teshuvah 4:4) states: one of the aveiros that are very hard to do teshuvah for is the aveirah of looking where one shouldn't. The Rambam writes that it is tough to teshuvah on this aveirah because "he doesn't know that looking is a grave aveirah because it leads to the aveirah of arayos, itself, as it states לְב ַבְכֶם אַחֲרֵי תָתוּרוּ וְלֹא ".עֵינֵיכֶם וְאַחֲרֵי
The Raavad (Baalei HaNefesh, Shaar HaKedushah) writes: "The first of all gates and boundaries is to guard the eyes, because when his eyes are guarded, his heart is guarded, and when his eyes and his heart are protected, his entire essence is protected. But if he doesn't guard his eyes, r"l, he will certainly fall and commit aveiros, as Chazal (Bamidbar Rabba 10:62) tell us, "The eye sees, the heart desires, and the body completes [the aveirah]."
