Some people think it is a “desire for sex,” that is, sexual activity or gratification (aka satisfaction of the feeling of being “horny”). But if what we mean by sexual desire is only a desire for sexual gratification, couldn’t that be achieved simply by masturbation? But solitary masturbation is so obviously different from sexual intercourse with another person! It is aimed at achieving the feeling of pleasure, but it lacks so many, and such important, dimensions of the experience of sexual union with another person! It seems to involve a desire for an outlet — but an outlet for what? The term “outlet” suggests that it is a second-best, an alternative to the real thing desired. That real thing desired is sexual union. (The desire is for a “who” rather than a “what” — unless you think you are “desiring” yourself sexually? Not likely!) Masturbation may involve a craving for sexual gratification of some sort, but it’s not really “sexual desire” as we usually use that term.
Sexual desire — this would come as a shock to a lot of people — is usually not wanting sex, but wanting a particular object, or rather, a particular person — it is a desire for intimacy and union with a special person of the opposite sex. What makes that person special can vary a lot. It can even be something very superficial and temporary and passing — but there is something there. Certainly physical aspects of the person are an important part of that. But it’s not just the body parts. That body is more than just a body. It’s a union of matter and spirit that we call a person. (And when a man uses a woman simply as an outlet, a way of obtaining sexual gratification, a form of masturbation, the experience is really diminished — if he were honest with himself, he would acknowledge that there is something missing, even if he pursues part of what he desires.)
In Lars and the Real Girl, the lead character Lars has a (non-sexual) “relationship” with a sex doll that is clearly the result of a pathology — the great difficulty Lars has in dealing closely with another human being. The doll (“Bianca”) ends up being a step on the path back to a greater capacity to deal with others. The premise of the film is that a sex doll is not a human being and can never be the object of true love, or sexual desire. People who use sex dolls are not desiring the sex doll — they simply want certain physical feelings of gratification or intimacy.
So sexual desire, as we usually understand it, is desire, not for sex, or physical responses, but for a person. This is the main point of Roger Scruton’s massive, complex, dense, and continually insightful treatise Sexual Desire.
