I would guess perhaps questions are asked. One example is the question to me as to whether I was a man who had sex with other men, or something of the sort, that is asked before you give blood. It is then probably assumed that because you were a man who engaged in homosexual behavior, that you are a homosexual yourself. That’s my thought anyways.
They also try to exclude men who have had sex with prostitutes (because of the danger of infection) but no questionnaire is going to catch everyone, even if everyone answers truthfully--a man could be infected from a girlfriend who earlier had an affair with an infected bisexual man. In the past there may have been an effort to exaggerate the extent of heterosexual transmission of the AIDS virus, but it does happen.