Donating blood is a friendly gesture by the public to give to those who are in need, but there are some restrictions in place to ensure that all blood samples remain healthy so that no one is accidentally given a blood-based disease, such as AIDS, which comes from the H.I.V. virus.
There was a long-standing ban on gay men donating blood due to the higher risk of contracting H.I.V. via unprotected anal sex, however in 2015, the United States's FDA made some changes. Now, a gay man simply needs to wait 12 months after having anal intercourse before they can be considered for donating blood.
These measures are in place because the anal lining is much thinner than the vaginal lining, and is more likely to tear during intercourse, which means the spread of blood-based viruses is more likely.
Once he has been cleared for not having the H.I.V. infection, he can donate blood just like anyone else. Modern blood screening techniques allow that blood to be tested for H.I.V. after the fact, and they are 99.9% accurate.
Why is there so much restriction on men who have anal sex and not women? It seems to come down to statistics and the FDA just wanted to play it safe so that no one is given H.I.V. positive blood. According to statistics, 72% of all new H.I.V. infections seem to come from gay men who partake in unprotected anal intercourse.
Of course, not all gay men have H.I.V. and it's impossible to contract H.I.V. from your partner if they don't already have it, so they shouldn't need to worry right? Nevertheless, even if a gay man doesn't have H.I.V., they are still subjected to the 12-month waiting period if they recently had unprotected anal sex.
Modern blood screening checks for much more than just H.I.V., but also for other viruses, such as West Nile, Syphilis, Hepatitis, and Malaria among many others.