There are wide individual differences in the content of peoples' preferred sexual fantasies. What determines the content that a person will find sexually arousing? According to one psychoanalytic approach, sexual arousal is inhibited by relationship anxieties, and the function of fantasy is to negate a person's key anxiety. Thus, the content of the preferred sexual fantasy should be predictable from the nature of a person's relationship anxiety. I report an initial pilot survey of fantasies and anxieties and then two experiments, one in which different anxieties are induced and we measure how arousing different fantasies are; the other in which we settle people into a fantasy and we see how that affects their anxieties. We find the matching between anxieties and fantasies predicted by the psychoanalytic theory stands up surprisingly well to experiment, but in another respect the theory needs to be stood on its head. Warning: If you are embarrassed by any of the following words you should not attend the talk or listen/watch the recording: penis, bondage, vaginal photoplethysmograph.