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Some people are predominantly heterosexual.
Some people are predominantly homosexual.
For others, it is more complicated.
In medical circles it has become common to refer to "Men who have sex with men" (MSM), which includes both actively (and exclusively) homosexual men, but also some men who perceive themselves to be "straight", and would object to being described as "gay". Many of these men have an active heterosexual sex life, but also have homosexual relationships - whether an occasional visit to a male prostitute for oral sex, or a more involved relationship.