It's relevant because we have to use terminology like MSM (men who have sex with men) in public health because otherwise we exclude people who do not identify as homosexual or bisexual. Often we don't care how someone self identifies. We do care about things they do.

Knowing why some men identify as heterosexual and refuse to identify as anything else -even though they have sex with men- can tell us about stigma and discrimination and can help us provide healthcare.

https://en.wikipedia.org/wiki/Men_who_have_sex_with_men