In his book, "Kuntres HaAvodah," Rebbe Sholom Dov Ber of Lubavitch, one of the early great Rebbes of Chabad, writes the following:
"Everyone who is concerned about his soul, not to pollute it, G-d forbid, should guard over his eyes. And if this is difficult for him, he should endeavor to restrain himself with all of his strength and might. He must take to heart that this matter is instrumental to the wellbeing of his soul, and if he does not guard himself in this matter, then all of his Divine service is accounted as nothing, and all of his achievements are as naught, and his service of G-d will fall lower and lower....
"Behold, there are people who are far from actually committing evil deeds, G-d forbid, but their hearts pull them to look and stare (at women). They gaze with a seemingly cold detachment, and they do not feel any immediate excitement when they look, but the reason for their being attracted is because they experience an inner pleasure... This gazing, even with seeming detachment, creates an impression and a great stain on the psyche, which will not go away without arousing some actual evil in its wake, G-d forbid....
"Thus, it is every man's duty to control himself and to guard over the things he sees. In so doing, he will save himself from evil, and his service of G-d will find favor. He will bring salvation to his soul, and he will rise higher and higher." (Kuntres HaAvodah, Ch. 2. For an English translation and commentary, see the book, "Love Like Fire and Water," Moznaim Publishing Corp.)