It's not. Risky male to female sex is also excluded (sleeping with anyone involved in the sex industry). It's a question of exposure to mixing blood, and thus communicating disease. Statistically, Male to Male sex increases that risk...
...The issue is that male to male sex by its very nature has a higher chance of passing on certain diseases than male to female. Risky male to female is also excluded (if you have slept with a prostitute or been in countries with high levels of disease)...
Arrgh! You cited the tamest statistic from the CDC article. Here are the others: 20% of gay men are HIV positive and 44% don't know it.
THAT is why the question is asked. As a future possible recipient of blood, I'm glad they ask it.
The question asked of heterosexuals =/= asked gay and bisexual men. The risk factor for straight people is risky sex, the risk factor asked for gay men is not risky sex, but sex at all - even once. Its not the same thing. If there was no bigotry behind the question it would be raised this way to all, no matter what orientation "have you had unprotected sex with more than one partner in the last 5 years, if yes have you been tested for HIV?"