I had a long, very rage filled rant typed out...but I decided to cut it down.
Basically, in the US (and a lot of other countries, but the US is the focus of this post) there's a lifetime ban on men who have sex with men donating blood.
The reasons for this ban are outdated and smack loudly of homophobia (considering the other "at risk" groups they don't ban).
The ban is a form of discrimination by unfairly targeting men who have sex with men, or effectively the gay and bisexual community. A permanent, blanket ban is instituted on any male who has had sex with another male even once since 1977 and without regard for his partner's HIV status nor for frequency, safe sex practices, or duration since. Yet if one has sex with an opposite-sex partner who is knowingly HIV-positive, he or she can give again in a year.
source Today (and tomorrow) the Department of Health and Human Services' Advisory Committee on Blood Safety and Availability is meeting to reconsider the ban.
Here's a link if you want to find out how you can email the Committee to urge them to revise the ban.
I have some other links on the ban, if you're interested, but they pretty much just say the same things these two do.