It’s a bit more complicated than that. In the early years of the HIV epidemic they at first didn’t even want to screen donors. The blood banks and the FDA were slow to introduce screening for a few reasons, one being that gay men were such good donors that a large proportion of the blood supply would have been removed. Eventually the risk became too great and do screening was introduced, just like we exclude those who were in Britain when TSEs were a risk. Note that these restrictions also never applied to lesbians, because they are not a high risk group.

40 years ago contracting HIV was still a serious, life threatening event. It’s also true that in the USA homosexual men represented one of the largest risk groups, unlike in Africa where other factors made spread between heterosexuals more common. It took hold in the gay male community due to the higher risk of anal sex, the popularity of bath houses, and the amount of sex men were having basically. Testing for HIV was also expensive. You could do it at the batch stage to reduce testing, but then you throw away a lot of blood. It’s only recently that PreP is widely available and used, so that HIV is more manageable (though it is still a serious illness).

My source for most of this is And The Band Played On, which apart from being one of the saddest books I ever read, outlines well the inaction by politicians, medical funding bodies, and even within the gay community itself, in tackling the epidemic. That it was allowed to happen is a black mark on the Reagan presidency.