This paper seeks to identify some useful insights microeconomic theory can provide into risky sexual behaviour. A neoclassical model of rational addiction and a behavioural economic model based on hyperbolic discounting are both applied to risky sex. It is argued that the phenomenon of risky sexual behaviour may best be described as a lack of self-control. Based on this, personal rules, which help linking current choices with similar future choices, provide a useful means of avoiding risky sexual behaviour.