The local all-women's race series (3 race series) has something like 10 times the female participation as your average coed race- sanctioned and operated by the same organization on the same courses. Certainly a different vibe (I've been a flagger) than the coed races, with a lot more newcomers participating. Much more of the tri/charity ride/recreational/fitness crowd out there. Although I'm a male and therefore fully qualified to offer an opinion on why the difference, I'm still a little confused why more don't show up for the regular races. I'd certainly love to see more beginner and intermediate women show up to fill out the fields, since it makes the experience better for all of the female competitors.
Certainly MTB and 'cross races see a larger percentage of women racers, with the 'cross series seeing exponential growth over the last few seasons. A lot of people cross over between the road and MTB scenes, and I don't see a great difference in attitude between the two. Type-A racer types are type-As no matter what width tire they're on. The fastest guys in both worlds are usually the most accepting of my feeble attempts to keep up, and the most supportive of all new racers- male or female. I'll have to ask some of the female regulars why they think women road racers are less common during the regular season.