MSM or M4M is a term for men who have sex with men, but who do not identify their sexual orientation.
Similar or related terms include, but are not limited to, the following:
DL or Down Low is a lifestyle predominately practiced by young, urban Black men who have sex with other men and women, yet do not identify as gay or bisexual. (Study published in the Journal of Bisexuality).
Bromance reflects the need to be emotionally close to another person, but without the need to be commited to a romantic relationship. Erikson described the psychosocial developmental stage faced in young adulthood as Intimacy vs. Isolation. He focused more on the role that romantic relationships played in life during this period, but as this new Millennial Gen playbook has illustrated, a wife and kids can wait -- close male friends are able to provide the intimate connections and emotional support that are needed.  ("Bromance and Tribe Identity," Psychology Today)