Netivot Shalom on Parshat Noah:
The beginning of the problems in the generation of the flood came, as the verse, states, "The divine beings saw how beautiful the daughters of man were, and took wives from among those that pleased them" (Genesis 6:2). The beginning of the error was through looking at that which was forbidden to them, "The divine beings saw", and even though they were men of repute, through "seeing", through looking, they became fundamentally flawed as we saw with the generation of the flood.
This follows what the Sages taught, "The eyes see, and the heart desires, and the tools of action complete the deed", that the beginning of the downfall begins with the eyes. From the "eye that sees" everything else follows, "each sin causes another sin" until we arrive at the point where, "The earth became corrupt before God; the earth was filled with lawlessness." (Genesis 6:11)
The flaw of the eye that see leads to a flaw of the mind, and once the mind is mixed up it no longer understands what is forbidden, even with regards to things that anyone should be able to understand are forbidden. This is also why it says "Noah found favor in God's eyes"; (Genesis 6:8) "In God's eyes", because his actions represent the repair of the eyes, because he guarded his eyes and sanctified them. Through this he merited to become a righteous man; (Genesis 6:9) he was able to reach the highest heights because he guarded his eyes.
להתרחק מן העריות ובו ז סעיפים:
צריך אדם להתרחק מהנשים מאד מאד... ואסור להסתכל בנשים שעומדות על הכביסה ואסור להסתכל בבגדי צבעונים של אשה שהוא מכירה אפי' אינם עליה שמא יבא להרהר בה. פגע אשה בשוק אסור להלך אחריה אלא רץ ומסלקה לצדדין או לאחריו ולא יעבור בפתח אשה זונה אפילו ברחוק ד' אמות והמסתכל אפילו באצבע קטנה של אשה ונתכוין ליהנות ממנה כאלו נסתכל בבית התורף (פי' ערוה) שלה ואסור לשמוע קול ערוה או לראות שערה והמתכוין לאחד מאלו הדברים מכין אותו מכת מרדות ואלו הדברים אסורים גם בחייבי לאוין:
A person must stay very far from women...He is forbidden to gaze at women doing laundry. He is forbidden to gaze at the colorful garments of a woman whom he recognizes, even if she is not wearing them, lest he come to have [forbidden] thoughts about her. If one encounters a woman in the marketplace, he is forbidden to walk behind her, but rather [must] run so that she is beside or behind him. One may not pass by the door of a promiscuous woman [or: a prostitute], even four cubits [around 6–8 ft or 2–2.5 m] distant. If one gazes even at the little finger of a woman with the intent to have pleasure from it, it is as though he gazed at her shameful place. If one intentionally does one of these things, we give him lashes of rebellion. These things are also forbidden in the case of ordinary Biblical prohibitions.