Shape D
You are right about STDs, safety should be the concern there and actions should be made with that in mind.
But the specific reference in this thread is to recieving oral sex and getting HIV. The odds are no where near russian roulette. In fact they are less than getting struck by lightning.
Realize the specific theoretical risk is situational. There must be significant bleeding in the mouth of the person performing the act. That is not a realistic situation for most. I can't imagine making that choice in that situation. Even if the person makes that choice there must also be a cut on the person recieving oral sex. Judging by the lack of a documented case it is reasonable to asume that people just don't do this.