The justification of banning gay men from giving blood is based on their increased rate of HIV infection in the United States. It isn't a ban put in place by any of the agencies which collect blood, the major ones of which (Red Cross, American Association of Blood Banks, and America's Blood Centers) have all requested that the lifetime ban be revised to a one year ban after sexual contact with another male-- which is the same as the ban on most other groups with a higher risk of contracting HIV.
The FDA, however, ultimately rejected their request, saying that at present, the screening process was not 100% effective, and that even if only one in a million cases of HIV failed to be detected (which is the failure rate for current screening tests), enough transfusions of blood products take place in the United States that a one in a million failure rate could lead to a significant increase in HIV+ blood entering into the blood supply.
If you'd care to read a more in depth explanation of the FDA's reasoning, you may find their official explanation here:
http://www.fda.gov/cber/faq/msmdonor.htm
To sum up, though, they essentially say they're not willing to risk it since gay males making an initial donation are anywhere between 800 times more likely than first time donors in the general population, and 8000 times more likely than repeat donors in the general population, to be HIV positive, and the tests still aren't infallible (no matter how insignificant their failure rate happens to be).
So, is it fair?
It doesn't sound like it to me. To be honest, I think they're grasping at straws. Yes, twenty new cases of HIV per year would be an unfortunate thing, and no one should suggest it isn't, but this is a group that's perfectly willing to allow surgical procedures with 20% and 30% mortality rates proceed because they figure the seventy to eighty percent of people they can save far outweighs the number that die in the attempt, especially if the only other option is to with-hold treatment and run a much greater risk of death.
To believe that's okay, and then throw a fit because your attempt to increase your blood supply to adequate levels comes with a 0.000001% chance something will go wrong, is a ridiculous double standard. All medical practices come with risk. On that front, it's a numbers game, and in this particular numbers game, one in a million happens to be absolutely golden odds.
I think the FDA is simply throwing that excuse out there as a pretext for continuing to discriminate and symbolically cut gay males out of the general population, to stigmatize them as somehow being sick or diseased, even though there isn't a medically compelling reason for them to do so... which isn't surprising given the homophobia of the current administration and its wont to interfere in the affairs of government agencies, especially scientific ones, for its own political and ideological ends.
In the end, though, it's their choice to make, not mine. I'm just one of the masses stuck living with the outcome of their decision.
Oh, and because the answers thus far have been a bit inaccurate when it comes to knowledge of blood (O+ blood is neither rare, nor the universal donor type), I have one more link, for those who would like to learn a bit more:
http://www.bloodbook.com/type-facts.html