The CDC writes on its website in an article (here) regarding new HIV diagnoses during 2015:

Gay and bisexual men are the population most affected by HIV. In 2015:

• Gay and bisexual men accounted for 67% (26,376) of all HIV diagnoses and 82% of diagnoses among males.
• African American gay and bisexual men accounted for the largest number of HIV diagnoses (10,315), followed by white (7,570) and Hispanic/Latino (7,013) gay and bisexual men.

And it provides the following graphic:

Considering that the population of gay and bisexual men is a small fraction of the overall population, the prevalence of HIV diagnosis among this population is extraordinarily high. That is to say, if we assume the population of gay and bisexual men is 2% of the male population, 82% of all new HIV diagnoses in the male population are among them.

The CDC noted (see here) regarding the data for 2014:

Gay and bisexual men made up an estimated 2% of the U.S. population, but accounted for 70% of new HIV infections.