Prevention approaches for reproductive health have evolved from an emphasis on individually focused models of behavior change to a recognition that risk reduction occurs within a context of social norms. Prevention programs can be improved by understanding how social structure influences sexual behavior and using that understanding to develop strategies for positive change. In a dynamic, urban context, communities are better conceptualized as informal networks of ties. These network structures may help to protect, or conversely, expose members to reproductive risk behaviors. Using data from a study of social and sexual networks conducted in northern Thailand, this article describes partner relations and social structure in the modern, urban context, and illustrates the links between individual, relational and structural properties and reproductive risk behaviors. Triangulation of ethnographic, survey and social network data collection and analytic tools provide an opportunity to interpret individual behaviors, meanings of relationships and structural properties of networks. Intervention approaches should build on existing networks, and address the complex meanings of romantic and sexual partnerships.