I do work in an area where women are extremely underrepresented--- open source software development. By some estimates, 1.5% of open source software developers are women (in contrast nearly 1/3rd of software developers with jobs are women).
I have noticed a few things. The first is that women tend to report being hit on fairly frequently just because the gender ratios are skewed. This isn't the end of the world and workplace/professional society romances and sexual encounters are are not that uncommon so it's going to happen unless we decide sex isn't part of the human condition (which isn't going to work).
The second is that the single most consistent annoyance mentioned to me is everyone asking "how do we get more women involved in open source?" This drives women away.
The third is hard to quantify but my sense is that of the women who come to open source, the vast majority come to it through employment. They have a job, and this is a means to get that job done. You see a lot more men who are independent consultants and hobbyists, and these make up the bulk of most projects. I would say the ratio of hobbyists/consultants of men to women in open source is probably 100:1.
If men get a job in open source it is more often through open source work, in other words participation is usually a career investment while for women it is more often something to do for a job.
There are exceptions of course. And one shouldn't underestimate their importance--- my grandmother was programming computers in Fortran, as she was an astrophysics professor. However at some point I think it is important to realize that asking the question, "how do we get more women involved" misses the point, and in fact drives women away. It's a counterproductive strategy. Simply saying "you are welcome at the table" is IMO a far more helpful way to proceed.