It's kind of a behavioral thing. Both women and men use hand gestures, but the big difference is that women's tend to be "looser." As in, they're not as forceful, there's not as much muscular tension, everything is lighter. Where with men's, there's more power behind their gestures... more muscular tension, more force, more directness.
Limp wrists are therefore feminine, because it's a very loose hand gesture where the hand is just allowed to flop around freely, loosely, which makes them look more passive. Where wrists that stay tense and tight show more force behind them, and therefore look more aggressive and male-ish.
It's not a conscious thing. It's just that women are taught to be less threatening, less direct with their motions, while men are taught to be more forceful and direct.
This is the same reason why loosely-curled fingers are more feminine, while fists are more masculine. They're the same basic hand position, but one is loose while the other one is tight.