Bigender (3.5e Trait)
From D&D Wiki
Your character can change gender at will. This takes 1 hour and is an Extraordinary Ability during this though you are considered helpless, it resembles the effect of a polymorph spell or similar effect. It is a total gender swap from appearance to all the other... 'special' bits.
Prerequisite: Must be taken at character creation (you will need to calculate weight and height for both of this characters genders)
Benefit: You can swap gender, pretty useful when disguising yourself. Acquaintances who meet the gender swapped you (who don't know you're Bigender) must make a Will save DC= 15 + your charisma modifier or believe they've never met you before. Once they find out its you they don't have to do this again.
Drawback: In some cultures Bigender people are treated as equals or even envied however, most of the time their killed at birth, so yeah people might have mixed reactions. Regardless of culture people are naturally distrusting of those so easily able to change their appearance and you take a -4 penalty to any diplomacy checks you make against those aware that you're Bigender. Additionally if you take something with a gender prerequisite then it only applies while you are the required gender.
Normal: You're a man or a women, simple.
Special: You're a man AND a women, simple.
Roleplaying Ideas: Being able to switch genders tends to foster split personalities, a bigender character may also have trouble identifying as any one gender or have been discriminated against in the past leading them to hide their ability possibly causing friction. Alternatively a more confident character may use this to their advantage essentially playing two characters and trying to prevent anyone from finding out.