I assume they wouldn't test the blood because testing might be expensive. It's probably easier and cheaper to ban gay men from donating than it is to just test all of their blood. If the statistics say that gay men have a significantly higher rate of HIV than the general population, then it makes sense. It sucks, but that's probably the best way to go. I would assume that blood is randomly tested, however.
All blood is tested.
You don't want to give someone hiv when giving them a blood transplant.
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I'm but a simple companion in the pack of gurus whereof SuperNiceDog is the pack leader!