Inappropriate Staring at the Opposite Gender

The basis for the prohibition of staring at the opposite gender inappropriately is the pasuk "ולא תתורו אחרי לבבכם ואחרי עיניכם אשר אתם זונים אחריהם. למען תזכרו ועשיתם את כל מצותי והייתם קדושים לאלהיכם" Chazal understand it to mean that one's eyes shouldn't gaze at inappropriately dressed women because it leads to sin.

1. It is permitted to stare at one's wife even when she's a Niddah even if it brings one's pleasure (nonetheless, it in some contexts it is still totally inappropriate). Nonetheless one may not look at certain parts of one's wife.