Every time you donate blood, you have to go through a lengthy questioning process, in which they make sure you haven't had a tattoo lately, or had an organ transplant, or that you haven't contracted any horrible diseases, or haven't been exposed to any horrible diseases, or may have been within sneezing distance of someone who may have once been within shouting distance of a horrible disease. And, if you're male, they also make sure you haven't had sex with another man. Ever.
This question always struck me as a little strange. The place I go to donate, they have a computer program doing the questioning, so there's nobody else in the room, and every time I get to hear a recorded voice say, "For males: from 1977 to the present, have you ever had sex with a man, even once?" in a very mild, non-judgmental tone.
The reason, of course, is AIDS. According to Wikipedia, a reliable resource if ever I knew one, the incidence of AIDS among men who have sex with men (or, as it's abbreviated, MSM) is sixty times higher than the general population. Of course, statistics are statistics, not arguments; all data requires context. Taking into account new screening methods, demographic shifts, and so forth, allowing MSMs to donate blood would probably be more beneficial than harmful.
I am not at all an expert on anything even remotely related to this debate, and I can't pretend to be one; however, I can make you readers aware that there is a debate. Heck, there's even a movie, Save A Life, on the subject.
When I was younger, I remember seeing a PSA in several DC comics about how AIDS isn't just "the gay disease" but can strike anyone:
(Incidentally, notice how the artist had to code these two men as "gay" through their clothes and hairstyles alone. I'm always interested in what signs artists use to identify aspects of characters that are otherwise difficult to depict.)
I'm kind of nostalgic for the days when superheroes did public service announcements—the blog Polite Dissent has a good collection of them. I'd like to see a superhero standing up for the rights of gay men to donate blood, though that might be a little too progressive for the big companies these days.
Also, according to Marvel (and, more specifically, Chris Claremont's Uncanny X-Men #159), while gay blood is probably fine, mutant blood is a no-go:
Anyway. Blood is fascinating, isn't it?