I think it was the other way around. In Japanese Final Fight she was a woman, but when they were porting the game to North America NOA wouldn't let them have a female enemy in a beat em up. To get around this Capcom labelled her a tranny instead, but that wasn't good enough for NOA so when the game was released in the States there was no Poison, just a male enemy named Billy. There have been conflicting statements since, but the most definitive one I can remember is the character designer for Final Fight saying "Poison is female in Japanese, and male in English" but then walking that back to "Poison is ambiguous in English".