A mathematical model is used to examine the effects of host population demography and transmission behavior on the evolution of virulence of a sexually transmitted pathogen such as HIV. The effect of the rate at which hosts acquire new partners is shown to depend critically on the details of the host population's growth pattern, sexual contact rate, and level of infection. At density-limited equilibrium, new partner acquisition rates have no effect on virulence. In an exponentially growing host population, higher partner acquisition rates favor the less virulent strain, as do lower rates of host population growth. In contrast, in uninfected populations, faster new partner acquisition rates encourage epidemics of the more virulent strain. Two extensions of the model--one including vertical transmission and another including within-host evolution--confirm the robustness of the predictions.