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 was 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.