I'm always sorry to hear the news about someone staying home for years and then deciding to go back to work. The reasons cited more often than not are economical. Of course, it may happen that one's husband loses his job or becomes unable to work, but it's not always the reason. Let's be honest, Western economies are not exactly doing fine right now, prices grow higher, various forms of subsidies people in Europe grew accustomed to are disappearing, medical care becomes more expensive, etc etc.
Does it mean that you can't live on one income any more? I'd like to stress that here I'm talking about middle class families. Even a lower middle class man will earn the amount of money which the working class couple normally earn with two of them (since most married women over here only work past-time) , and yet there are enough of these couples having children and living just fine. Which means, if they can do it, you can do it, too! As someone pointed out to me, often it's not really the question of needs unmet, but rather about preserving a certain lifestyle.
When the family finances start going south, the temptation is often strong to try and keep up appearances, even if it means the wife and mother getting a job outside home. Middle class couples often have a certain standard to maintain, which becomes more important to them than anything else. Of course, it's my opinion, that even an average working class couple could live on one income considering the fact that they still qualify for some government freebies, but it would take a great deal of good management.
On the other hand, when money is not so tight, it's easier to become careless and start spending it like water, which will always ruin you in the end, no matter how much you earn. I'll just give you a personal example. I normally received a certain sum of money from my husband for weekly shopping and I managed to save enough that I could afford to get out and lunch with my friends from time to time.
We really didn't need to economise so I asked him to give me more money. My idea was to save it for hobby purposes (yarn, fabric, knitting magazines) and I was quite enthusiastic about it, however, after a couple of months I noticed that by the end of the week I had less money left than before, when the amount I received had been smaller. I just started buying more expensive brands and overall became more careless, which proves that if you don't exercise tight control on how much you really give out, you'll come into trouble.
If the family are committed to living on one income, they should take some tough decisions beforehand, including where they are planning to live, which school the children will attend (commuting is expensive), which car they are going to drive etc.
I remember reading a story in comments section of a newspaper about a guy who said that though his wife was a SAHM, by the end of the month they had more money over than his two-income relatives who were pretty resentful about the fact. He rented while they had a huge mortgage, he didn't need to pay daycare for his kids since his wife was home, he paid less taxes since he was the sole provider and thus lower income, his family didn't eat out so often, his vacations were probably not so luxurious and yet he and his wife were happy while the other couple were bitter. Priorities are everything!
In the end, it doesn't depend so much on what your husband earns, but rather, on what's more important for you. If it's your conviction that a married woman's place is her home, you'll do everything possible to make it happen. If you believe otherwise, well, it's a free country, after all. For those falling in the first category, I'd like to recommend this great article which also gives detailed suggestions on how to reduce expenses so that you, too, could stay home:
How Can I Afford To Stay Home?