I am so glad you brought this theme up, smyoung, because I too have pondered this as well for the longest time, since my school days. I do have my own theories and some have already been highlighted but here is my take on it:
When I was in school, our ID dept was on the same floor as the Interior Design dept. Not too long after befriending a few interior design majors and always passing by their studios, did I notice 99 percent of the student body being female (and as someone else noticed, impeccably dressed and made up EVERYDAY!). Many of the young women I befriended were mostly clueless as to what ID was (as most lay people are) and most of the time, they were under the misconception that it was engineering-based or mechanical in nature. The name "industrial" threw most of them off because it gave connotations of "industry", hence "too mechanical and industrial-gritty for me". When they came to realize the aesthetic nature of ID (color, texture, form.... aspects they admired within interior design as well) and the problem solving involved, some were pleasantly surprised they had not gone in the direction of ID vs. Interiors. I know, because I also thought I wanted to do interiors at first (I am male) and hadn't heard of ID at the time. I soon came to realize after careful analysis of my sketches that my emphasis and attraction was always focused on the actual objects within the rooms I was designing and not the rooms themselves.
I have personally witnessed throughout my 4 year stint in school, several interiors majors in our ID classes who enjoyed the experience of building models, sketching, etc... especially if the projects involved furniture, chairs, lighting, housewares, appliances, etc. Two young women I know of actually switched mid-way through their education. When talking to them in depth, I came to notice that they too thought they wanted to be interior designers, like I did, but then realized they were most attracted to the three-dimensionality of ID, home decor and housewares, etc. I am still baffled more women are not involved in ID because not only do women mostly make purchasing decisions in the home and are more involved in how their homes look, but most of the products that they themselves enjoy are designed by men, who sometimes have no clue as to what women need. As we all know by now, most design geared towards women is a "shrink and pink" job. We truly DO need more women in ID and a female perspective. The same goes for fashion, which is baffling: why do women rely on men to design for them?!
I think ID needs more exposure in high schools and in the media. Anyone notice that even in TV sitcoms or dramas, when a character is involved in design, it's always either fashion or interiors? Even reality TV hasn't caught up! All I see are competition shows dedicated to remaking rooms or fashion-related. Ugh.
ID certainly is slowing gaining a reputation, with the advent of internet, blogs dedicated to design, etc... so I predict soon there to be a change in gender demographics within our profession. It's only a matter of time.