The American Red Cross actually favors allowing men who have had sex with men to donate after a 12 month deferral. It's the FDA that has insisted on the lifetime ban. The guidelines appear somewhat arbitrary, seemingly based on some late-1970s conception of which groups are "bad people."
Consider John, who traded blow-jobs one time with his college friend in 1978, but since then has been in a monogamous relationship to a woman. The male/male sex was 36 years ago, but John is forever barred from donating blood. Now compare John to Mary: Mary traveled a lot on business, and it took her a while to realize that her husband was having anonymous, unprotected anal sex with men. It's been a little over a year since Mary last had sex with her (now ex-) husband, so she's free to donate.
Then there's the whole issue that it's all on the honor system...