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"Not only is physical contact with women prohibited, gazing at them to enjoy their beauty is also a serious Torah transgression." (Shulchan Oruch, Even HaEzer, Ch. 300, Section 100:2)

Regarding the Torah commandment, “You shall not go astray after your hearts and after your eyes, (Bamidbar, 15:39) the Talmud explains: “After your hearts – this refers to idolatry. After your eyes – this refers to sexual immorality.” (Berachot 12B)

"The eye and the heart are the two instruments of sin. The eye sees, the heart desires, and the body completes the action." (Rashi on Bamidbar, 25:39)

Today, because of the promiscuity and immodesty that pervade modern life and Western culture, we have become accustomed to this state of affairs, as if this is the natural way to be. However, Jewish Laws demands a higher level of moral behavior.

“Whoever gazes at a woman forbidden to him, and says to himself that there is nothing wrong with this, for he hasn’t had sexual relations with her, or even touched her, he is mistaken. Gazing at women is a serious wrongdoing, for it brings a man to sexual transgression, as it says, You shall not go astray after your hearts and after your eyes.”  (Hilchot Tshuva of the Rambam, 4:4)

The Gemara teaches: “You should guard yourself from every evil thing – this means a man should not gaze on an attractive woman, even a single one, nor upon married woman, even if she is ugly.” (Avodah Zora 20A.)

This is because, “A man shouldn’t have sexual thoughts in the day and come to seminal impurity at night.” (Avodah Zora 20B.)

See Rabbi Eliahu Leon Levi's Kabbalistic explanation of the prohibition of gazing at women.