It really is all about HIV and implied risk.
The two main modes of transmission of HIV is through unprotected penetrative sex and IV drug abuse.
In 2011, there was an estimated 96.000 people with HIV in the UK. Prevalence is around 1.5 people per thousand in the general adult population, but that increases drastically in that cohort of the population Men who have sex with Men (MSM) - it rises to 47 cases per 1000.
HIV can have a very long latency period, where it lies dormant within the body - residing principally within the white blood cells.
Our ability to detect HIV infection has improved markedly over the last couple of decades, from an Antibody test, to an Antigen test and more. Even the most modern and sensitive HIV tests are not infallible however.
This does mean that a unit of blood can be donated, can be screened and found negative, but still have the potential to infect the recipient.
It is a simple fact therefore that MSM donors carry the highest potential risk of HIV transmission. A declaration of celibacy for a 12 month period does reduce that risk a great deal.
So - It is an example of an exercise of the precautionary principle. Not meant to be offensively discriminatory, just a fact of life.......
HIV may no longer be viewed as a death sentence as it once was, with the use of a cocktail of antiviral drugs, and its transmission can be largely prevented by the use of safe sex - but it is still a serious risk, and the NHSBT have an obligation to their recipients to ensure that the blood supply is as safe as possible....
As for donating whilst considered underweight - application of the precautionary principle again. Someone underweight can ill- afford the donation of a unit of blood.