Playing with a four-foot-long cock has to be a lot of fun. But an encounter with a djinn might be very frustrating. You could give him a full-body stroke job, but what could he do for you in return? If he tried to jack you off, he might end up ripping your cock off by accident.
Maybe if you made him cum, he'd grant you a wish in appreciation. Perhaps this is where the idea of rubbing the genie's lamp originated and was sanitized by puritans. You're not suppose to rub his lamp; you're meant to rub his cock.