A project carrying an "open source" or "free software" license is not necessarily an "open source" project. Plenty of "Cathedral" projects with paid developers with an open source license that may (or may not) get downstream patches kicked up. Those projects are going to look like any other corporate development group. These are really the core projects.
The "open source projects" of people hacking code make up the bulk of developers in open source, and is the hobbyist developers. People that have a lot of time to devote to a hobby are either single, or older empty nesters. Men can hang out in the single realm and start a family @ 40, women cannot. This limits women from engaging in serious time commitments like open source projects.
The pool of women available to do this is pretty small.
That's without dealing with the fact that women tend to have tighter deviations from the norm in various areas, which means that any group that is selected from extreme outliers is going to be disproportionately male. This is true whether you are selecting politicians that reach Federal office, people that are extremely interested in programming to pursue as a hobby, moving to America as a day laboring immigrant, or criminally oriented men to form a gang. The outliers are predominately (but not exclusively) male.
In local politics, where the time commitment is NOT as extreme and the skill set needed to be elected is NOT that extreme, we have a pretty good mix of men and women on city counsels, school boards, mayoral seats, etc. Not 50-50, but a pretty good representation. We have plenty of female mayors, but we've NEVER had a female governor. Outliers in general are predominately male.