Just force all blood donors to get tested for infection, regardless of orientation, then give the clean ones a certfification with expiry. Re-test as required to continue donating.
Back in the 80's, one of the things we learned from the opening stages of the AIDS epidemic is the possibility that a new disease agent will enter the human population, sight unseen. If such a new virus were to appear, it could spread silently for years before being identified (just has HIV did).
It is this risk which had led to the exclusion of the gay population. The elevated risk for HIV infection in that population serves as a marker -- it demonstrates that they have the epidemiological risk characteristics to become the initial host for such a new disease, should it ever appear. By excluding higher-risk groups, the idea is to slow down the opening stages of the next epidemic.