I think the idea is that there are two types of qualified women.
The first type is the one in your 1:20 ratio. She's competent and can tolerate the environment of a male dominated workplace. She's not the issue.
The second (theoretical) type is also skilled (or at least has the right inclinations and intelligence type), but she's uncomfortable with the male dominated workplace and so she either leaves the field early or never even gets into the field to begin with.
Some believe that the remedy for the second type of woman is seen to be a place where there are more women, period. This allows them to have friends and the ability to have a more balanced environment. The increase in women in general will make it more attractive to the skilled women as well.
Obviously, this is an assumption, but not a necessarily a terrible one. Many people only feel comfortable among people like them. Same goes for gender, skin color or ethnicity.
A lot of this comes down to what the actual value of a more equitable ratio actually is. What are the quantifiable benefits of this sort of parity or diversity? And are those benefits come at the expense of productivity or opportunity for those who are not selected purely on the basis of their gender? Does one benefit outweigh the other? If so, then the feelings and misconceptions of the other side should give way, at least to the extent that the greatest benefit can be achieved.
I think there is a lot of shooting from the hip on this. I'd like someone to tell me:
1) Does having more females in IT being a perceptible benefit to either IT, or themselves?
2) What methods are necessary to achieve those benefits?
3) In the end, do any benefits actually outweigh the costs?