Sexual desire is very simple and straightforward . . . and very complex and elusive
Men tend to experience it as: "I want . . that."
But what is the "that"? What exactly is "sexual desire"? It seems so obvious, but it's just not.
One notion of sexual desire might be "desire for sex" (aka being "horny"). But if we mean by this a desire for sexual gratification, what about masturbation? This is a response to desire for sexual gratification, but is it "sexual desire" for oneself? Not likely. It's a desire for sexual gratification of some sort, but not "sexual desire" as we usually use that term.
Sexual desire is usually not a desire for sex, but desire for a particular object, or rather, a particular person. And a person is different from a sexual object in general.
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 desire their own feelings of sexual gratification.
Sexual desire, as we usually understand it, is desire, not for sex, or sexual feelings or sexual responses, but for a person. This is the main point of Roger Scruton's massive, and continually insightful, treatise Sexual Desire.