I doubt it's a myth. Many gay men of all races have been known to date and marry women in order to avoid the possible raft of society.
Elton John, Rock Hudson, Vincente Minnelli, Anthony Perkins, Cole Porter, and Oscar Wilde, are just a few famous gay men who married women at one time. They didn't call it "down low" then.
Hopefully with "marriage equality" laws and more acceptance of people in the LGBT community there will be less people who feel the need to live in the shadows. They can be who they are.