If we are using a model like presented in Molyneux's Practical Anarchy, then the social norm could be that people would be expected to carry some kind of certification of health from a trusted authority. When Marv gets tested for HIV and it comes back positive, that authority would take away his certification, or at least update it to be HIV+ (after all, HIV+ people do date, but they may not jump in bed with someone else who also has hepititus, etc). A person might say that it would be up to the woman to inquire about this certification and it would be up to Marv to be truthful about that inquiry. Thus a person could have the view that if a man or woman is too interested in sex to inquire about the health certification.