From a website about epidemics...

"At this time, data suggest that gay, bisexual, and other men who have sex with men make up the majority of cases in the current monkeypox outbreak."

Who falls into the third group that isn't already covered by one of the first two?

I guess the author was just leaving room for definitional expansion. Remember when there was no such term as "LGBTQ?" I still don't know how to pronounce it!