According to philosopher Seiriol Morgan, sexual desire isn’t desire in the strict sense in which desire is a longing for something you don’t have, for example a car or a girlfriend.
Sexual desire involves bodily pleasure and certain mental states or processes. It is more akin to emotions, such as love and caring, than it is to desire in the normal sense of the word.
What may look like pure physical arousal, or raw animal instinct, is in far the most cases much more complicated.
Two people having sex anonymously with pieces of cloth over their heads may look like a case of pure physical pleasure but the thoughts of the anonymity likely contribute to (or subtract from) the sexual desire, in which case the sexual desire is not purely physical.
Sexual desire in this (partially mental) sense can be morally wrong, even if everyone involved consented to the act in the fullest sense. It can be wrong if, for example, the thoughts directed toward the other person are morally reprehensible.
In a scenario from the film Dangerous Liaisons a charming and good-looking man, who can get almost anyone he wants, starts to get bored because everything is so easy for him, so he plans to seduce a woman who despises him because of her virtues.
His sexual desire, in that case, is fueled by the thoughts of her willfully abandoning her moral principles. This kind of sexual desire may be thought to be morally reprehensible, not because it involves deceit, but because the thought-components of the desire are morally criticizable.
Above perspectives was summerized by Berit Brogaard D.M.Sci., Ph.D - a Professor of Philosophy and the Director of the Brogaard Lab for Multisensory Research at the University of Miami - from Morgan, S. (2003) “Sex in the Head”, Journal of Applied Philosophy 20: 1-16.