You touch on something, but I think there are deeper issues at work here.
(FWIW, I welcome their involvement in OSS. There is quite a bit of historical social machinery that stands in the way and someone needs to do something about it.)
Every few months (YMMV), it seems that there is a story on /. about a lack of women in science and engineering. Some posters on this particular topic have also suggested that very few women pursue "geeky" endeavors in their free time. From a very arm-chair/anecdotal position, I think these both have some of the main root cause: from a young age women are not encouraged to pursue technology or science in the same way boys are.
There are many factors for this, it's not as simple as saying that "the parents are doing a bad job" or "it's the schools". Like most things, it's not that black and white. As a boy, for a variety of reasons, I spent many hours reading books, tinkering, and generally "being by myself" that led to how I solve problems and spurred me on my way in my interests. My sisters, under the auspices of a very liberal, slightly disconnected intellectual, were given the same sort of options, but followed their peers more: socializing, etc. But I cannot rightly claim that this is what happens to everyone. What I can say is that it does seem like even children are encouraged in different paths by the whole of society and that this is hard to fight, but should start somewhere. Did my teachers encourage males to "be nerdy" and females to be social? Are there different pressures exerted on young girls, not just by their families, but by media?
If we take that disparity between male and female at an academic level (that is, the difference in enrollment/matriculation under science and technology by the sexes) and then envision those graduates as working professionals, the numbers make more sense. If (these are purely made up numbers to illustrate a point) 75% of graduates in, say, Computer Science are males and only 20% of graduates go on to contribute to OSS, there is a good chance that the make up of the OSS contributing graduates will be predominately male (there is no guarantee, of course... it could well be that that 20% is part of the 25% of female graduates in my made up scenario, but ceteris paribus you'd not expect that).
I don't think this has as much to do with salary as it does these other social rules and the existing social frameworks that exist. That is why groups like the Ada Initiative may seem backwards to some, but are needed. Someone needs to encourage the young (and old) women on the fence that they can contribute to OSS, that it's okay to be geeky. Someone needs to set these examples for girls so that they don't fall into the age old traps of misogyny.
Additional food for thought: I do many technical interviews and I see very few females who contribute to OSS in them, but a sad majority of the men are often quite bigoted and not as liberal as they would like to believe. That is to say, anecdotally, there is sometimes a correlation with OSS work and poor empathy skills which result in these types of problems (groping, etc). Sometimes this social outsider "dive into books" sort of thing that may contribute to the division to begin with, also makes some men who's social skills are undeveloped (to put it nicely) and pathetic (to put it bluntly).