A voyeur is a person who derives sexual gratification by watching the naked bodies and/or the sexual practices of others. Usually, the voyeur watches from a secret vantage point.
Voyeurism is a sinful act because it violates the privacy of a person or persons and because it can also include observing sexual relations between two people. A voyeur who watches a naked person or persons without their knowledge essentially "steals" from them that privacy and intimacy intended only for the self and or the person's spouse. Even if the voyeur has the consent of the other(s), it is still sinful since true nakedness and sexual relations are only to be enjoyed between a husband and wife. Furthermore, voyeurism is a lustful action, something that is denied by Scripture.
Matt. 5:27-28, "You have heard that it was said, 'You shall not commit adultery'; 28 but I say to you, that everyone who looks on a woman to lust for her has committed adultery with her already in his heart."