Poison is either a cross-dresser [which is unrealistic lol c'mon look at her] or a hermaphrodite [makes more sense] in Japan.
But in the US I'm pretty sure she's a transsexual. Then there's the people that take Svensson's word and just look at her as 100% woman because he said her gender is supposed to be ambiguous, which means open to any meaning.
I see Poison as a transsexual because I feel that it's the best choice, adding much variety to the character. I've seen that video before and I like how the guy explained that if she was decided to be 100% woman, she would not be near as interesting as she was now. If she had been 100% woman from the start, I doubt she would have been in SFxT.