I've worked at a bloodbank briefly during my pharmacy school time. The HIV is just one of the problems with male homossexuals. Backdoors sex can lead to a thing called bacterial endocarditis that causes some problems in your heart and this carries trough blood donations (this is also related to poor dental hygiene).
But as you can imagine, this also is related to females who enjoy the other way.
That's why during interview they ask about your sex life, the problem isn't really about homosexualism, but due to sexual habits.
But then again they discard lots of female sex blood parts (not the whole blood, just some components) due to fetal eritroblastosis related causes, so its not just them who are "affected"
Blood transfer is a really serious issue, and even with all the exams the blood passes trough there is still lots of things that go undetected. I've read reports about people contracting chagas disesase trough blood donations.
Dang, I coudln't even donate blood myself due to acne when I was in college.