I don't think people necessarily 'look down' on SAHMs, but rather it's not so much the norm these days. Typically families rely on more than one income financially, so many women who see themselves primarily as SAHMs also combine that with some paid work, squeezed into the week somewhere.
Actually now you've said that, I think there is a kind of reverse discrimination for not working, because some people think that because your family can afford for you to stay home, then you must have money kwim? which is bull****, it just means that we are able to survive on one income and are no different to the families that need two incomes coming in - no different to if one parent stayed home while the other worked two jobs.