The poll results have slowed down, so this might be a good time to take stock.
It's true that, in the aggregate, women do more housework and child care than men do. It's not necessary to cite a source for this. The sheer number of SAHMs compared to SAHDs would alone be enough to cause the discrepancy, even if all other households showed an even distribution. Again, I'm talking about the aggregate. I'm not talking about the dynamics of individual households; I'm saying that if you take the total hours of housework and child care performed during a year across the entire country, more of those hours are performed by women. By quite a lot.
As of the moment I'm writing this, the poll shows a 60/40 split in favor of women doing more housework and childcare than their partner. (More women than men cast votes, but when I accounted for that fact, the calculation came out the same.) This doesn't prove much of anything at all, since the poll is not scientific and was asking about comparisons rather than raw numbers. But it's fair to say that the poll results are consistent with the above observation that women do more housework and child care than men do (again, in the aggregate, as stated above).
What does this mean? It means that "women do housework and child care" is a cultural norm. We see our parents performing to this norm, we see it in our peers, we see it in media and entertainment. It comes to seem "natural" that women do housework and child care. It's a "thing women do," like shopping for clothes and drinking cosmopolitans.
If a man in an opposite-sex relationship acts as if housework and child care is "not his job," there's likely not one single reason (he might be lazy, he might not know how, he might be an asshole, etc.), but this norm is probably a factor.
Here's the other thing about cultural norms: they are self-reinforcing and self-perpetuating. We're all familiar with the norm of hyperconsumerism. People buy stuff, everybody sees people buying stuff, people buy more stuff. Performing the "women do housework and child care" norm is the same. By performing it, we perpetuate it. (And I'm including myself here; I do far more household work than DH does.) Whether you think that's good, bad, or neutral depends on how you feel about gender stereotypes.
This is probably the point where people start freaking out about crazy feminists who are telling people they shouldn't be SAHMs. I'm not saying that. If you think, "being a SAHM is the right decision for my family," and also think, "I would prefer not to perpetuate norms that reinforce stereotypical gender roles," then you're going to have to pick one, and which one you should pick is pretty clear. Martyrdom is pointless. We all live in the world we live in, not the world we wish we lived in.
This problem will probably go away, if slowly. Norms are powerful, but we do have the free ability to flout them (see hyperconsumerism, above) and they do change.
ETA: I used hyperconsumerism as an example of a norm. I also used "women doing housework and child care" as an example of a norm. I'm now afraid someone will accuse me of saying that women who take care of their children should get facepunches. Um, let me be clear that I do not think this. I was only trying to illustrate how cultural norms are perpetuated.