I once debated with myself whether I should donate blood given that I'd had sex with men before, but whom I was sure were HIV-negative. I did a quick Fermi estimate looking at the amount of contaminated blood samples the blood bank could expect nationally, first with only heterosexual donors, and then with heterosexual + homosexual donors, given that each blood sample underwent the most accurate HIV tests. The results were pretty staggering (order of magnitude difference).

That convinced me that the proscription was there for a very good reason and that I shouldn't violate it.