Tribpot - Yes, the underlying assumption that childcare is a woman's job to fund is so prevalent. I've picked people up on it. A woman who told me she loved her job recently said to me "Of course, it's not really worth it because I only take home £ each month after childcare, tax and my travel card." I said, "but you said you loved your job". She agreed, so I said, "Well, what is left from your other half's job if you take the childcare off his job?" She had never done the calculation because, even though she loved her job and she wanted to work, it had literally never occurred to her that the childcare came off her husband's salary, or should be shared.
The reason that is dangerous is that she had a running line of guilt that her children were in childcare for "only £x". It made her value her own working low. I bet not many men, whether they are second earners or not, sit around struggling with that type of angst. There is an underlying assumption that, if a man wants to work, he will. If a male second earner who wants to stay home then great, but rarely would he suffer any criticism if he chose not to.
I also think that these things should be done with more information. It so easy to do the simplistic calculation of salary X minus childcare Y =Z, and to think one shouldn't/doesn't have to work if Z is too low. But it is very hard to access any information on what the long term cost of not working is, in terms of ease of re-entry to your area, pension, earnings impact.
A slight side track, but I know a lot of women my mother's age who have grown up children and have spent a boring 20 years working in low paid work after their children left home. Or who haven't worked and have gone a little crazy - a friend's mother used to phone her every day when she started university because she was so bored and lonely. It is very hard to re-launch a successful, and more importantly interesting, career late in life after a long gap. That frustration can be a hidden cost which is also hard to factor in. Being a SAHM with children at home is a very different prospect to being a SAHM once they have left.