What pisses me and other WOHM off is the assumption from some SAHM that they are in some way superior because they are slaves to their children 24/7. I see this, but I think most people feel that other people think they are superior (even though most people are not). A lot of SAHM think WOHM think they are superior because they're earning money and using their brains.
I'm with TwoOddSocks. I have (temporarily, I hope) given up a career I love because we can't afford for me to go back to work. I do most of the childcare and housework because I do see it as part of the deal (and I have time at the moment, due to age of the children; they are not school-age yet, but do sleep through), but I would hate to receive an attitude from my husband that everything should be done, just so, at these times (not saying this is your attitude OP; as your children are at school the situations are entirely different).
I actually agree that domestic work that you would have to pay for in [the] absence [of a SATP] is work. My DH and I certainly see it that way; I currently see myself as "earning" as a wage what we would have to spend on childcare, cleaner and commute, were I to return to work.
Not to say that some of what I do is not a pleasure - I am lucky to be home with my children in many ways, which is another reason I take on more household chores, so that my DH can enjoy his time with them at weekends instead of doing washing. (And by the way, caring for small children when another adult is in the house to bring you a towel, or or prepare lunch, or watch them while you have a shower, is not the same as doing it alone!)
But sometimes it would be nice to be able to switch off when"home" from work, watch what you like on TV or go for a run because you've earned the downtime, not feeling you should do "one more thing" (mop the kitchen floor), knowing that someone has bought something for dinner (even if you're going to cook it), and that if the children wake in the night someone else will take care of it. I think it's the lack of parameters in SAHP that can be stifling, the feeling that you're always on duty, and also the sheer unrelenting monotony. Not that YABU to expect SAHP to do more than half of housework, but this is why they might be moaning about it.
HTH!