No but as they will never have nine months pregnant or push out a baby a two, I think it would be wise if they were, unless he was able to look after the children and take sick days instead of you when dealing with terrible case of morning sickness, which in that case, do as you please.
Also the other issues is, with time off taken by women during pregnancy and child rearing, they loose a lot of money in superannuation.
But again the husband doesn't have to be the main money earner, however what else will he do to keep himself feeling "worthwhile" or whatever it is that does drive them to work. Unless of course he has lots of hobbies or something. There is a lack of stimulation when raising children and women always complain of it that they didn't have a decent conversation with anybody today. I think this is a bigger question that it seems at the start with a lot more factors to consider than I originally thought.
For part of our marriage I was the main income earner, while my husband studied and then consequently looked for work.