What's the deal with the sexually dysfunctional characters in One Piece? Well, my friends, you've come to the right place.
Luffy - what's the deal with this guy? Sexually immature, or has he not hit puberty yet? He was floored by Nami's Happiness Punch for sure, but when someone like Sube Sube Alvida says she's in love with you, your response should be something other than "I don't care." But regardless, Luffy is probably the most normal crewmember, sexuality-wise.
Usopp - his sexuality is completely devoted to Kaya. Fairly normal, like Luffy.
Zoro - Okay, the first one with serious issues. Zoro seems the type who's soured on women, perhaps due to his frustrated feelings towards Kuina at a young age (being unable to defeat a girl, etc.), which seem to resurface around Tashigi. These messed-up feelings about women could lead to homosexuality. Zoro would probably enjoy rough mansex. If he had a woman, the sex would probably also be very rough. It's the dysfunctional connection in his mind between swords/competition and emotion.
Sanji - promiscuity is sometimes a sign of sexual abuse at a young age. [Insert perverted jokes about Zeff here.] Or perhaps Sanji chases every woman in sight as a way of disguising his possibly bisexual tendencies. Maybe Zeff had a youth experience that led him to revere women; this was passed on to Sanji, who also found women irresistible.
Chopper - He's fifteen and a reindeer. Leave him alone.
Nami - the Dominant. Just look at the way she's always beating on the men, asserting herself, acting defiant, and that enormous phallic symbol she carries around.
Robin - the Submissive. Her tendency towards handcuffs, the "I'll do anything" she had to do to survive, her generally shy nature. Not to mention her suggestive Devil Fruit ability.
Spandam - this guy definitely has issues about beating women. He probably has a sexual inferiority complex which leads him to abuse his power and position over others.
Lucci - RAPES WOMEN. HE'S A SADISTIC BASTARD.
Kaku - actually seems like he would be shy around women. But they probably love him, though.
Jabura - another guy with an inferiority complex! Tries WAY too hard around women.
Califa - bisexual dominatrix, hates men, possibly due to a bad experience with them in her past. Enjoys causing pain to men and women alike.
Any others? Add your own!