Everyone watched and cheated
In keeping with the comparison to Russian roulette, the people would watch the revolver as it is passed around and the trigger is pulled. Once someone actually fires off a round, they know there are no more bullets in the revolver, and are safe.
In the same way, people would wait for another person to have sex. If that person ends up having an STD, they know that the woman he fucked is infected as well, and will stay away from her.
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