From whence came wench, and other slanderous slurs?

From the 11 October 2007 edition of Nature:

A 'hussy' was once a perfectly respectable housewife, and 'wench' just meant 'young woman,' but both terms now connote a woman of loose morals. And 'lady'- once used just for a woman of noble birth- is now the standard term for any woman.

Intriguingly, words for men generally don't suffer the same fate, and sometimes even improve their connotations ('knight' originally meant just a boy or retainer). Parallel patterns have occurred in other languages (for example, as with the German Weib, which suffered the fate of 'wench').

So, commonly used words that simply mean woman take on ever more slanderous and sexualized meanings until they eventually become offensive slurs, while words that refer to common men tend not to transform into insults over the centuries, but actually tend to turn into words with positive connotations. Why might that be? The Nature article continues:

The most obvious explanation for this phenomenon is that language users (or at least those who have historically been responsible for recording language - men) are consistently misogynistic. But a more convincing 'invisible hand' explanation invokes a simple individual rule: when talking to or about women, err on the side of politeness. Given two options, on normal and one more polite ('hussy' versus 'lady'), this rule, if applied widely and consistently, leads to 'lady' becoming the common form. 'Hussy' or 'wench', by comparison, become ever-less polite over time. The best intentions lead to pejoration as an unintended consequence.

This 'people err on the side of polite' explanation fails to address two questions- why the same does not happen with words referring to men (one supposes that people tend to use the more respectful terms when referring to unknown men as well), and why words like hussy and wench took on sexual meanings as well, rather than just seeming to denote poor or low-brow women?

As Arnold Zwicky of Language Log notes, we might be witnessing a rapid modern day pejoration of a common word. The word "moist," he writes in his October 25th post, is becoming widely seen as an offensive word. Zwicky cites the experience of one University of Georgia English professor:

A student in my Shakespeare class announced that the word "moist" (which I had uttered to describe Egypt in Antony & Cleopatra) is offensive to women. Some of the other women in the class concurred (not hostilely--just as a matter of information for a clueless male professor). I was somewhat flabbergasted, and nobody would articulate a reason for the offensiveness--except for one male student's eventual suggestion that the word reminds women of sexual arousal. That association is not at all beside-the-point of my description of Egypt in the play--but why would such a connotation make the word offensive per se? As far as I could ascertain, "damp" and "wet" don't carry whatever stigma attaches to "moist." What am I missing here?!

I am with the professor on this one. While moist might very well be a favored descriptor in adult lit, it is surely used with equal frequency in cooking magazines. Certain adjectives are also frequently used to describe male arousal, yet these adjectives out of that context do not connote anything scandalous. So how is it that common words can so rapidly pick up sexy or offensive meanings, but only when referring to women?