OP - you said you are soon to go back to work, so I'm assuming at end of maternity leave. have you said anything about being nervous, about being worried about settling your DC into childcare, worried about leaving him/her? Because then the 'lucky' response would make a lot of sense!
They are 'lucky' to not have to go through that.
However, it doesn't mean they have a rich husband. Just one who's wage can cover the bills with a bit left over. Those bills might be considerably less than yours because they've planned to be able to have this choice. We could have borrowed nearly 50% more than we did when we bought this house (when I was pregnant with DC1), the bank would have lent us what we thought was a huge amount, but DH was very clear, we should be able to live off one wage, whichever one it was. As it is, that was very sensible, as having DC2 meant my wage would have been entirely taken up with childcare and commuting costs. (realistically, we couldn't afford DC2, and I do know someone who stopped at 1 DC because they couldn't afford to drop to one wage and her wage would have been wiped out by childcare costs).
Oh and yes, our marriage might break down. Of course at that point, having not worked for 3-4 years will mean my degree/other qualifications and decade of work experience will suddenly disappear off my CV and i'll not be able to get any job at all, plus the equity in the house will magically vanish, the savings and shares will disappear, and everyone knows no one has ever taken the decision to be a SAHM while their DCs are little and then managed to get another job in the next 20-30 years of their working life after their DCs are a bit older... 
Look round the 60-something working woman retiring now, most will have had a few years out of the work force raising DCs, it's not a 'forever' decision unless you want it to be.