GandTea- great post. But the last sentence is not always correct- it is a choice. Many parents could do with one salary if they wanted to, and cut their cloth accordingly. But the expectations re houses, designer furniture, cars, holidays, clothes, etc, mean that often 2 salaries are required for such choices for many.
But there is another dimension- and that is if both in a couple have worked terribly hard at their studiesm, then terribly hard at getting to a high level in their career- which they totally both enjoy- then young women these days are less likely to say 'ah well, I'll give up my career and ambitions, because I am the mother' as before- and not all men, however modern in their outlook- will agree to swap- unless they have a problem with their career themselves. In some jobs, both can go part-time and share childcare- but that is very rare, especially in managerial roles.
Imagine 2 doctors- both studied for many years, then spent years working very long and stressful hours and are now both Consultants. Which one will give up their job? And why? Are we still saying, in the 21st C, that mother should- or that she should become a part-time GP?
Many will choose to send children to good quality nursery, and then work even harder to be there for the kids when they are not working- and honestly and truly- it works wonderfully well and the kids really don't suffer. There is no way we, as grand-parents- could have given the social contacts, activities and stimulation, etc, our GCs got- full-time- even though we are young and active grand-parents and me an ex teacher.