Do you have a precise meaning of "unscientific" in mind?

I mean that they maintain practices, justified on scientific grounds, that are blatantly illogical.

If someone did a double-blind randomized study comparing disease incidence in countries that did or did not accept blood from game men, I'd very much like to hear about it.

Laying aside that blinding and randomization aren't really necessary for statistical studies, I'd much rather see a study that compared the relative amounts of blood contaminated with sexually transmitted diseases across several countries with similar demographics and cultural trends, some of which refused to accept blood from gay men.

But we don't always get the evidence we want, sadly, and so we must make do with what we have, much as the U.S. Department of Health and Human Services must.

Or, we can ignore the evidence entirely and look at whether HHS is even being consistent, which is an easier question to answer definitively. The current policy is that any man who has had sex with another man since 1977 is banned from donating blood for life. It is also current policy that any woman who has had sex with such a man is banned from donating blood for the next year.

I will leave identifying this failure of rationality as an exercise for the reader.