There are a lot of not-altogether-scientific reasons. Gay people can’t give blood. If I recall correctly British people can’t either because of Foot and Mouth disease, something that happened in the 90s.
They are based on reasons that are statistically founded in some way because not everyone's blood can be tested as extensively as anyone would like, so reducing risk factors helps. I can't speak for every country but this is at least the case for the ones I've lived in and donated in.
Also, it's no longer true in many (most?) places that "gay people" can't give blood, as the distinguishing factor nowadays is usually recent sexual activity.
Note that "recent at-risk sexual activity" specifically means (among other things) a man engaging in sex with another man in the past 12 months (at least that's the case in Australia[1]).
Practically speaking this means that the vast majority of gay men cannot donate blood -- even if the criteria don't explicitly say the words "gay men cannot donate blood". Don't get me wrong, I agree that there are purely statistical reasons why these restrictions are in place (blood banks aren't out looking for excuses to reduce their supplies) -- but it's really not accurate to say that it's no longer the case that gay men cannot donate blood.
I also hasten to note that having many sexual partners is not considered "at-risk sexual activity" (though there are some exceptions), while two monogamous men in a homosexual relationship having sex is always considered as being "at-risk".
> In 2015, the US Food and Drug Administration (FDA) relaxed its guidance but still required men to abstain from sex with another man for at least a year before they could give blood, a policy that the the American Public Health Association said was “not based in science”.
Can confirm: I'm not allowed to give blood in Australia because of the small risk (with a potentially very bad downside if it manifests) that I'm infected with mad cow disease, due to living up in the UK in the 90s.
They still let me sign up to the organ donor list though.
Edit: foot and mouth disease was a decade later. I don't believe it's a problem in humans?
Also, it's no longer true in many (most?) places that "gay people" can't give blood, as the distinguishing factor nowadays is usually recent sexual activity.