Perhaps they are the heterosexual men being raped in the prisons, and now (sadly) in the military.
Good point about the prison rape. Also, “hustlers” (male prostitutes) do not necessarily consider themselves to be homosexual.
From the other end of the spectrum, there are likely many (or at least some) men who have homosexual leanings; but are in a monogamous relationship with a woman.
All scientific research (including medical research) requires very precise operating definitions. In this instance, it’s the actual behaviour that matters; so “MSM” is much better than “identify as gay or bi-sexual”.