system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: since AIDS is no longer considered a “homosexual disease” why do blood banks still ask if a man has had sexual contact with another man, on the questionnaire?
gpt
While it's not still considered a "homosexual disease" HIV/AIDS is still very prevalent in the homosexual community. Gay and bisexual men make up 70% of all new cases of HIV, despite them being less than 1.7% of the population (assuming 50% of homosexuals and bisexuals are male). 

This is partly because of behavioral tendencies of gay/bisexual men who tend to have more anonymous partners and have unprotected sex. Also, anal sex is more traumatic to tissues than vaginal sex. The rectum is not built to accommodate sexual intercourse like the vagina is, so there is a higher chance of the HIV virus entering the bloodstream via anal sex than vaginal sex.

So the question for blood donation is, "Are you a male who has had anal sex, even once, with another man?" Saying yes may disqualify your donation as this behavior has a significantly higher chance of having HIV than other donor types. Yes, they do test blood for HIV and Hepatitis, but these tests are reliant on antibodies. If you were infected within the previous month, you could still be infected but it wouldn't show up on a test.