He's definitely a male but is androgynous and effeminate in his body language, tone of voice, and sometimes his choice of attire. These are typical traits to give a cunning lead villian whose powerful so can afford to be arrogant and vain. Its a long running question of whether he's gay or bisexual or is straight and merely vain (like Vega in Street Fighter) but likes to creep people out or this is the way he is. It definitely is a psychological factor he uses to his advantage since he is a great temptor. He leads one to believe he can offer all things. He has the devil's charm, and thats carried across through the whole snake theme.
When he changes bodies his character is still the same, so we know its really nothing physical, though we do know that he prefers the trim, refined look. Of all the bodies he could possess he possesses handsome young men or beautiful women. He doesn't go for the muscle-bound type or the physically dominant. He chooses bodies which suit his strengths and ones he can get the most out of. When we see the flashback episodes, we see him in his original body (or what we can presume is his original body) and he has the snake-like features then too, as a little boy.
The whole idea of him 'shedding his skin', like a snake, is a reoccuring theme.