on the DL, Down Low, Down-low, on the down low etc
Down Low has its origins in African American slang, where it refers to black men who have sex with other men, as well as with women, but who do not identify as gay or bisexual. This term is well known. In August 2003 the New York Times Magazine ran a cover story called "Double Lives on the Down Low", written by Benoit Denizet-Lewis. Several episodes of The Oprah Winfrey Show were also dedicated to the subject including an episode aired 16 April 2004 and titled A Secret Sex World: Living on the 'Down Low' ; the show featured J.L. King discussing his book 'On the Down Low: A Journey into the Lives of "Straight" Black Men Who Sleep with Men'.
"Of course he's on the Down Low, he's black."
See urbandictionary.com for several defintions of this term.
This entry was emailed to me by Dan Blocker, who observed the lack of 'culturally inclusive' euphemisms in my collection. He sent several terms originally used by African American and Latino communities in the US.