MSM is the acronym for: Men who have Sex with Men. Not homosexuals, or bisexuals. Just men having sex with men, the end.
The term MSM is often used in social research and medical literature. The term is used in favour of homosexual or bisexual males to include all men who have sex with men regardless of identity.
MSM directly refers to sexual activities between men. It is useful to have this distinction as a homosexual or bisexual identified man is not necessarily sexually active with men. And Gay, homosexual, bisexual can refer to a lot more than just sexual activity.
Additionally it has been found that many straight identified men do indeed have sex with men.