Men who engage in human sexual behavior with other men, but do not necessarily identify themselves as "gay" or "bisexual" are often described as MSM or Men who have Sex with Men. Likewise, different definitions of "homosexual" may include or exclude people with varying levels of activity, frequency, or interest.

Some members of the urban street cultures refer to this behavior as being on the "Down-Low". Ex-Convicts/Thugs/Gangsters/HipHop cultures frequently use this term. While an MSM's male partners are often aware of their alternate sexual activities, wives and girlfriends are rarely aware of these other activities.

In general, MSM is seen as a wider category that covers both self-identified gay men and other men who engage in same-sex sexual activity.

In MSM relationships condoms are often not used, contributing to the spread of STDs. Health campaigns and officials often seek to target self-identified "straight" or "bisexual" "men who have sex with men" or "men who like sex with men" (MSM) in addition to self-identified "gay" or "bisexual" men.

Some men who identify themselves as heterosexual who are in a relationship with a woman sometimes take jobs in the porn industry in gay porn movies.