All this discussion got me thinking about this. The fundamentals of what I am talking about might not be related to the other discussion from some people's perspectives, but I do think it prompts us to think about this as a bigger picture issue.
I believe that flexibility, support, making science and the family workable, needs to be a two-way street. It needs to come from the bottom up as well as the top down.
I believe that mentors/employers owe their mentees/trainees/employees the right to flexibility and support as long as their productivity is maintained, and indeed that by offering those to their people, they give them the space TO maintain that productivity.
I also believe, however, that mentees/trainees/employees owe their mentors/employers that same degree of support and flexibility. You should be ready to give the same support that you expect from others.
Discrimination and prejudice are well-documented, prohibited by law and pretty thoroughly discussed with regards to hiring, recruiting, treating your employees. University (and other) policies are in place to protect trainees from being discriminated against, and to require their employers to support them for personal issues like medical and family situations. But nobody talks about the other side: students/postdocs/trainees/applicants unconsciously or consciously discriminating against young/pregnant/female/minority mentors or employers. You as the trainee need to make sure you are going to get the best possible opportunity, right? You expect that a mentor and institution owes that to you, right? And yes: we do, because that is our job and it is why we are where we are.
But when it comes to decision time and the top candidates are choosing whether to join a lab run by an older faculty member with no family committments (no kids or grown kids), who is on the top of the ladder in their field vs. a young assistant professor (promising, but maybe she's pregnant and you worry about how much time she'll be able to devote to your needs)... ISN'T THAT THE SAME DECISION as whether or not to hire a young, pregnant woman to work for you wondering if her productivity might suffer because of her situation?
WE NEED EACH OTHER TO SUCCEED. No, of course I'm not advocating that every trainee should perform bottom-up affirmative action in this way. But young and/or pregnant and/or minority assistant profs need you excellent trainees to believe in us just as much as you need us and the old greybeard moneybags to believe in you. And we ALL have to work together to make science more open to the people who keep getting lost out of the pipeline AT ALL LEVELS.