The giving-blood thing probably isn't worded as well as it could be, but it's basically just about statistics. In the UK, around 40%
of the people known to have HIV are men who have, at some point, had sex with another man (regardless of whether they happen to identify as gay or not, and many don't). Sure, that still means that less than half of the people who have HIV are gay men (especially as gay men are probably more likely to go for HIV testing, so more of them will actually know their HIV status), but still, statistically, it's a high-risk group. I agree it's unfair, I don't like it either, I know it doesn't address the trans issue, or that of hetero people who have unsafe sex, but at the end of the day safety has to be the top priority in matters of public health, and that's primarily a statistical thing. If at some point in the future the numbers are different, and being a man who has had sex with another man is no longer such a high risk, hopefully they'll change the guidelines.
Sorry if i've offended anyone by saying this.
What I should have added is that I was very much in agreement with what y'all are saying, until I read this article
by the Terrence Higgins Trust. In fact probably i should've just posted that link rather than trying to explain it in my own words! I don't know how well-known THT is outside the UK, so i'll just explain that it's a UK charity set up by gay men for gay men in the early days of the AIDS epidemic, and they do a lot of great work both in sexual health promotion and gay advocacy. It surprised me a lot that even they'd be supportive of the current rules (although recommending that the policy be reviewed regularly - which i also agree is a good idea), but it made me rethink the issue a bit. I hope that explains things a bit better.