I called CDC and managed to ask a couple questions they couldn't answer, so I got elevated. They're going to e-mail me, and I can follow up with them by e-mail.

"Over half (53%) of these new infections occurred in gay and bisexual men" is correct, but my questions were:

How did they know the infections were new? (Were there previous negative tests?)

Were members of other risk groups included in the 53%, i.e. IV drug users?

They aren't going to do anything more than speculate as to why the difference between the US and other places in the world are so different. It could be because there's a lot of gay/bisexual men who are HIV positive already in the community. In Africa, there may be a lot of heterosexuals who are HIV positive already in the community.