The role of sexual exclusivity (monogamy) in relation to HIV and the use of rule making in non exclusive (open) relationships are discussed. Data from interviews with 387 homosexually active men are presented. The most common sexual/relational configuration amongst these men is that of open relationships. Sexual non-exclusivity is found to be associated with longer relationships, and a greater age difference between partners. The strategies some of these men are using to maintain sexually non- exclusive relationships are outlined. These rules pertain both to interpersonal dynamics and HIV prevention. Differing epidemiological significance of the rules and some implications for health education are discussed.