I believe floppy wrists are *quite* natural; it shows you are relaxed, but it also does make a person more vulnerable, which is probably why society encourages it for women but discourages it in men. It is, I believe a societal thing.
I have found that the more I remove my "male" mask, the more my wrists flow naturally and gracefully. My walk becomes linked to my emotional state... *everything* begins to link to my emotional state. I think that's a big part of the mannerisms divide... men are taught to hide/repress their emotions; revealing emotional states again implies weakness.
Our society tends to reward women for being vulnerable (more revealing clothing=less protection, high heals=vulnerable because the wearer is off-balance and unlikely to be able to run quickly, Showing emotion=easier to take advantage of, no pockets=carrying a purse that can be taken, wearing a skirt=less protection from sexual attack, etc) and men for being as invulnerable as possible...
At least, that's my current working theory. Oh, and limp wrists are much easier to break than firm ones.
One way of looking at it is that this allows women to be more easily controlled and potentially victimized. Another way of looking at it is that women are expected to be untouchable and therefor to have no need of defense, to be free to express themselves as they wish. I, at least, greatly prefer the latter.