Sexually transmitted infections (STIs) can be found in a range of hosts. Their epidemiology is predicted to vary with mean and variance in number of mating partners and, in more refined models, contact and social structure. Weak dependence of mating rate on host density leads to a prediction of density-independent dynamics, including the possibility that sterilising infections could drive their hosts extinct. Infection’s impact on the host is predicted to select for mate choice against infected partners and reduced mating rates. We examine these predictions against STIs in nature, with a particular focus on studies of beetle–ectoparasitic mite interactions. The Adalia bipunctata–Coccipolipus interaction has given rich insights, with ease of scoring infection and mating activity in natural populations enabling detailed documentation of dynamics. Laboratory study has allowed precise estimation of transmission parameters to inform models and focused analysis of behaviour. These studies have confirmed the core impact of mating rate on STI dynamics, but revealed unexpected drivers such as food supply (positively driving mating rate) and sex ratio (enhancing spread and producing male-biased prevalence), alongside constraints on spread from host phenology.