Poison is actually a case of complete androgen insensitivity syndrome
Complete androgen insensitivity syndrome
is an actual condition in which a person will have male chromosomes, but develop physically female. Poison is always depicted with a female body, including very large breasts, and in recent games is voice acted by a woman. Despite Capcom's inability or unwillingness to outright say what Poison is, this would make more sense than Poison being a cross-dressing man or undergoing Easy Sex Change
surgery- and being realistic, a low-level street punk with no money and a probable criminal history probably isn't paying for any extensive surgery. CAIS would likely better explain the gender confusion without breaking current canon or going against her character artwork.