@Picassospaintbrush I think you're right! That quote about middle-aged women was possibly more accurate in the past...or for mothers in their 30s or 40s who feel unable to leave their (perhaps flexible and/or part-time) job that works well for the family, but less well for them personally, so they don't want to rock the boat.
Someone at my work has been part-time since her daughter was born (5+ years ago), and despite official policy encouraging managers to put everyone forward for promotion once they've been in a role for a certain amount of time, those working part-time seem to have to fight for it, and still are denied year after year...she has now given up trying. 
Also, I'm sure it's been said before, but it seems like especially from what I've read of that facilitated man thread helpfully linked earlier the child-related issue for women often starts with the firstborn (after taking up to 12 months off for maternity leave, during which time their partner is still working, may be on track for promotion, etc.) and then if a second child arrives, it becomes much harder financially (or even impossible logistically) for the family to make it work...and by then, the mother's career may be stagnating or she's earning less than the father, so the logical choice for the family is for her to give up work to avoid those childcare costs altogether, or at least go part-time for a few years, and the father's salary and career becomes more important to the family, with the pressures that brings.
If people only had one child, maybe it would match up better with the saving up to take 1-2 years off travelling scenario (assuming that the parents can afford high quality childcare for 10+ hours per day with their combined salaries once they both go back to work full-time...), but it's not really the same situation.
And even if it were, this mother penalty apparently only accounts for around 20% of the pay gap (not sure how they determined that though -- I still need to read that paper I linked to earlier)...
But I suspect young women and men do, to some extent, make early career-related decisions with these 'traditional' roles subconsciously in mind -- well before they think about having kids or getting married.
P.S. I'm still interested in the approaches companies are taking to combat this, if you can share.