There was a TED talk by the CEO of facebook recently and she emphasised how women tend to think about starting a family and position themselves for that some years in advance. It becomes pretty obvious to any postgraduate student that, due to the long hours culture, combining academia with a young family will be hard - and that the system will also penalise them for taking maternity leave. The same applies to other jobs with long hours cultures (how many women aspire to be traders?).
If you are feeling broody at 23, the endless uncertainty of finding a few postdocs here and there is unapealling. Even if you know you are not ready to start a family, other career paths can mean you can see yourself as reaching that positon within a couple of years.
There is a huge amount of luck involved in academic success. Citations breed citations and once your name becomes known, you are cited more. If you have a family in mind, and I think women tend to think this way at an earlier age, it might be too much of a gamble.