How much of that is that we (1) socialize women to be that way; and (2) impose disproportionate parenting obligations on them to begin with, which causes them to seek more flexibility?
I cannot speak to (2), but cross-cultural studies suggests that differences in the way women are raised across different cultures don't influence women in the way you suggest. In fact, in more prosperous societies, there are greater sex differences, not less: https://www.youtube.com/watch?v=l3JuaIg99X0
Everything that's been said in this thread follows from each commenter's position on this question.
In the world of policy, we often have to run with an assumption before we can prove it. The question is, what presumption do we apply? The logical thing to do is to make an educated guess from observation. Look at history: Women have long been excluded from the professional world, and been socialized to accept non-economic societal roles. This is recent history: Had my grandmother gone into my field (law), she would've started her career at a time when firms openly refused to hire women regardless of qualifications; had my mom done the same, she would've started at a time when the practice was still widespread and the federal government was fighting to stamp it out. We can also look cross-sectionally. In countries with strong communist influence, the representation of women in science/engineering is much higher than in the U.S.: http://knowledge.wharton.upenn.edu/article/stubborn-obstacle....
If you're a doctor treating a patient who had cancer 10 years ago and it went into remission, you're committing malpractice if that's not the first thing you check for if new troubles arise. Given the above observations, it is disingenuous to start with the presumption that observed differences have biological rather than sociological explanations.
There are all manner of research findings about the biological differences in the sexes. And reasonable people can in good faith build cases from that research just as you've done from the observable evidence.
Would I be wrong to conclude that you have a strong preference for the socialization explanation? I feel like this debate is held back by the illogical and erroneous belief that a socialization explanation is Good, while the nature explanation, should it turn out to be true, would be Bad. Or, that anybody who believes the nature explanation is themselves Bad or at the very least has bad intentions.