Really? I know what a beard is, as do most people. Straight people, gay people, young and old. It's not a deep dark secret thing. It was really common in Hollywood in the 40s-50s, as being Gay was not acceptable to the general public, and it would have ruined actor's careers.
MC didn't know, because he was extremely sheltered in terms of media access, only listened to music, had no TV, and couldn't afford to go to the movies.
Are you telling me that you never heard of what a "beard" meant for gay people?