I’ve heard the phrase “keep it on the down low” a lot, and I’ve never heard it used with any connotation of homosexuality. I first remembering hearing the phrase in the movie “old school”: Will Ferrel is modifying his car and tells his wife to keep it on the down low since its not exactly street legal (or something like that). The phrase obviously does not have any sort of sexual connotation in that line.
I wonder if I am alone in being oblivious to the switch in usage from a general sense of “secretive” to a specific sense of secretive homosexual activity. As far as I can tell, the shift towards it specifically connoting secretive gay sex arose in the African American culture, just as the term “down low” arose there in the first place (this is a bit of a WAG, but not entirely: I am inferring this from a review of several websites that didn’t come right out and say this but that seemed to imply it). My further WAG would be that non-African Americans are beginning to catch on in the shift in meaning but many are still oblivious to it, just as I was until reading this entry.
Also, as far as I can tell, the phrase is still used in the broader sense of keeping (anything) secret, but the newer sense seems to have largely overpowered the older sense, at least among those who are familiar with the new sense of the term. Of course, context, as aleays, is important, and I doubt anybody who heard the line from “Old School” would think that Will Ferrel is, for some reason, referencing secretive gay sex. But, for those who are aware of the newer sense of the term, it seems to be the “default” sense in which it used, unless the context clearly demonstrates otherwise.