AIDS has invigorated and distorted the study of sexual behaviour. Because that study began so recently, there remain many unanswered questions about why we have sex at all, why we do sex one way rather than another, or even how we define sex. Yet in every instance in which well-designed and adequately resourced behavioural interventions have been implemented, these have netted success in the form of failing HIV incidences or prevalences. But, despite these successes, such interventions remain patchy and poorly supported. Perhaps humankind's traditional aversion for the public discussion of sexual matters underlies this reticence. Or maybe a new era of 'creeping absolutism' - in which biomedical advances are given premature credit for what they can achieve in HIV control - has arrived.