actually, she was conceived as a "new-half" from the very beginning
*video*
after digging some more I come up with the conclusion that not even Capcom know.
in the original "Final Fight" Poison creator Akira Yasuda states that was designed as a female and during game development was changes to futa for "fighting females is bad" reasons thanks to the planned release of the game in USA, still that wasn't enough and was replaced in the US version of the game, remained with minor censoring as a cross-dresser in EUR regions, and thus she was a female in Japan and a male in the rest of the world.
in "Final Fight: Revenge" s/he was a playable character and Capcom stated that "might have gotten a sex change" and the profiles in the 2005 "Capcom classics compilation" confirmed that was a transgender.
later in an interview with "Final Fight" producer and Akira Nishitani stated that "the viewer to decide."
in another interview to Street Fighter 4 producer, Yoshinori Ono, he was asked about it and added to the mix "Let's set the record straight: in North America, Poison is officially a post-op transsexual. But in Japan, she simply tucks her business away to look female.". later the asked about what
female characters could be added to Street Fighter 4 he said that would be confusing due to region-specific genders.
during the development of Street Fighter X Tekken he addressed what he said and said that Poison was a post-op transsexual but later he simply stuck to what Akira Nishitani said that was up to the player to decide.
Conclusion: nobody knows and everybody win(?)
as for me, is a she because I like it that way and nobody can say I'm wrong
info from here
EDIT: I'm still angry by the rule34 images