The number of bi-sexual males may be larger than you imagine.  

Other than infected needles, how else do you think many of the women in the US and particularly in Africa contract HIV?  

Toilet seats?

Not according to the CDC.

"Gay, bisexual, and other men who have sex with men made up an estimated 2% of the population but 55% of people living with HIV in the United States in 2013. If current diagnosis rates continue, 1 in 6 gay and bisexual men will be diagnosed with HIV in their lifetime, including 1 in 2 black/African American  gay and bisexual men, 1 in 4 Hispanic/Latino gay and bisexual men, and 1 in 11 white gay and bisexual men."

"Gay and bisexual men accounted for 83% (29,418) of the estimated new HIV diagnoses among all males aged 13 and older and 67% of the total estimated new diagnoses in the United States."

 https://www.cdc.gov/hiv/group/msm/index.html