Not true. But it did start in prison.
"While sagging did gain its start in the U.S. prison system, it was not a clothes-wearing style authored by imprisoned homosexuals intent upon advertising their interest in casual flings. Sagging pants became the behind-the-bars thing thanks to ill-fittingprison-issue garb: some of those incarcerated were provided with clothing a few sizes too large. That oversizing, coupled with the lack of belts in the big house, led to a great number of jailbirds whose pants were falling off their arses. (Belts are not permitted in most correctional facilities because all too often the lifeless bodies of their inmate owners have been found hanging from them.) "