The Rambam (Hilchos Teshuva 4:4) says that feasting one’s eyes on arayos could block the road to doing teshuva. The person may be comfortably telling himself: "I’ve never really done anything wrong. I never approached that woman." But how faulty his reasoning, and oh how willingly is he being led to disaster. Besides for the fact that one thing leads to another, the words ואחרי עיניכם - after your eyes (Bamidbar 15:39), mean that even just looking is forbidden.
Arayos is a broad term. Anything connected with or leading to arayos, including histaklus, falls under the category of abizrayhu de’arayos (an accessory to the transgression) and becomes one of the three prohibitions for which we must be prepared to surrender our lives. And so,the cunning Satan isn’t only out to seduce one to commit a forbidden action. To acquire a bulging bag of loot, he only needs our eyes. With plenty of histaklus material always available, he could have his victim slapping onto himself an adhesive label, inscribed in indelible ink, with one word - rasha. A thorough teshuva, though, will immediately rip that ugly label to shreds.
Feasting the eyes on forbidden sights can sometimes be as destructive as committing actual adultery. The Midrash (Midrash Rabbah, Vayikra 23:12) quotes the passuk עין נואף - an adulterous eye. Chazal say (Brachos 12b) אחרי עיניכם זו זנות - that merely following one’s eyes can be considered an act of adultery. Why is this so?
The answer is that the eyes are windows through which the soul absorbs images and scenes from the outside world which then become part of our personality and the building blocks of our imagination. Gazing at something weaves threads which connect the viewer to the viewed and binds them together. Once we have seen something, we are never quite the same again. An indelible imprint remains inside us forever.
Sponge-like, our soul imbibes whatever diet our eyes feed it. Will it be clean, kosher nourishment or will it be unrefined injurious substances? If what our eyes soak up is toxic, then that’s the kind of fare our precious neshama will ingest. Thus Chazal (Megillah 28a) warned against looking into the face of a rasha because of its impact on our soul.
Unlike our other limbs which perform a given task after which it’s over and done, the eyes work like a conveyor belt that keeps on delivering from without to within.
Incidentally, eyes can function in the opposite direction, too, exuding from within to without. When Moshe Rabbeinu climbed up Har Nevo and looked down longingly at Eretz Yisrael, his eyes gazed at it with such heartfelt goodwill that they dispensed a unique quality of wisdom still potent today.
Bilaam, on the other hand, tried to use this power for evil. His intention was to spit forth a stream of venomous curses, but he knew that in order to activate the evil, he would have to gaze upon his victims. So he, too, climbed up a mountain to look down. Such is the power of looking, be it for better or for worse.