While we're far from the AIDS-scare in the eighties, being gay and engaging in sexual intercourse with other men puts you, statistically speaking, in a HIV risk category that's
significantly higher compared to people who do not.
http://www.cdc.gov/hiv/group/msm/
Excluding certain groups further decreases the risk of transmitting HIV through blood transfusions. It's not a way to judge you for your sexual preferences, it's medical risk reduction based on proven statistical data.
I mean, I totally understand that it feels like discrimination and it is, but post-donation blood testing is not a 100% guarantee and mistakes do happen. Measures are taken at
every step of the blood-donation process to reduce risk.
But let's get back on-topic, shall we.