A complete graph on N vertices has (N^2 - N)/2 edges. A complete bipartite graph on N vertices has <= N^2/4 edges. There are just more sexual interactions possible among non-heterosexuals, and men who have sex with men have more (relatively) dedicated social gatherings than women who have sex with women. The number of people who congregate in a particular event is generally unaffected by population and instead determined by the size of the venue.

Anal sex is a risk factor for certain diseases, particularly HIV, but monkeypox does not require genital contact (or significant fluid transfer) for spread, so it's not clear if condoms would make much of a difference in this case.