We develop a heterosexual partnership network model based on Russian survey data. Our purpose is to study the spread of sexually transmitted infections (STIs) such as HIV and gonorrhea and to assess prevention strategies. The key ingredients of the model are age-structured sexual behavior, casual versus steady partnerships, a core group, and condom usage. In the first part, we attempt to reproduce the observed STI prevalence as a way of validating the model through correct parametrization. In the second part, we study possible prevention strategies. The results show that the rate of spread varies according to how many individuals use condoms and how many core group members there are. An STI can disappear from the population if it disappears from the core group, and increasing condom usage in the core group has disproportionate impacts on STI prevalence. The model predicts that HIV prevalence is highest in the 25-34 age class.