"Those who forget history are doomed to repeat it" - George Santayana
"Woman" derives from the combination of the words wif (lit. woman) and man (lit. human being). "Man" , on the other hand, used to be "wer"*. Going to werman in the same pattern that took wif to wimman, then getting dropped just to "man".
Why? Most likely because, over time, the assumption that wer was the default sex/gender, that you only had to specify gender when talking about the other gender. This has actually already happened in Esperanto, a language only 100 years old (based on language forums- I don't know Esperanto myself)- so clearly whatever caused it to happen isn't out of our systems yet. The distance between "man", a gender non-specified term for people, ended up being uneven- reflecting the same idea that "generic he" carries.
How to rectify this? Some people, for reasons I genuinely would like to hear about, suggested removing the man (gender non-specified person) from woman- ending up with womyn/womon, while allowing the wers of the species to go on using man without a prefix. I'm sure there was a very good reason for this. I'm also sure I don't like that idea and would rather re-attach the prefix. Or a prefix. I'm not attached to 'wer' I'm just not creative enough to come up with a better one. (or- I'm too creative and would come up with something positively miserable as a prefix) I also like the similarity between our modern woman and werman- although that might be not so nice to other genders, who generally don't include 'man' in their title.
I like this idea, though. Phrases such as "All men are created equal" regain (or take on) a neutral meaning (that should have been there even if it wasn't). Rather than reinforcing the sexism of those phrases by further distancing women from 'man' and 'human'- we can hopefully reinforce the fact that women and wermen have the right to be included in the words "man" and "human" equally.
*That entry on 'man' has a lot of text. Here's the relevant bit: "Sense of "adult male" is late (c.1000); O.E. used wer and wif to distinguish the sexes, but wer began to disappear late 13c. and was replaced by man."