I don't doubt that such sexism exists, but even were it to disappear completely, I still don't believe men would be interested in teaching kindergarteners in any significant numbers. Were I to consider a teaching job, my enthusiasm would drop as the student's age decreased. I like younger kids just fine, but I sure wouldn't want to spend all day, every day teaching them. I'm just a sampling of one, of course, but those rates makes perfect sense to me, and it has nothing to do with others' perceptions.
You seem to be implying that were the sexism to disappear, the ratios would be significantly different. I'm curious how much difference you feel there would be? It's obviously just speculation on my part, but my gut feeling is that it might make for a few percentage points of difference, but nothing dramatic.
I think our efforts are better expended trying to stamp out racism and sexism, improve the economic situation so as to better afford equal opportunity, and then let people work in whatever job they damn well feel like taking on, instead of according to some social-agenda schedule. I'm pretty sure a lot of people advocating this sort of affirmative action have the best of intentions, but I feel it's trying to fix the symptoms rather than the problem. Even worse, when you're talking about the different interests of men and women, you may end up trying to "fix" an imbalance that occurs naturally because of our inherently different interests.
Why is it so hard to believe that men and women might be interested in different careers or have different interests? Do you think it's because of artificially stamped "gender roles" that society imposes on men and women? Doesn't it make sense that our societal mores and roles simply tend to align with gender-specific traits and talents evolved over the last hundred thousand years or so? Those traits, of course, are largely driven by biological realities, so unless men start giving birth, it makes no sense for them to compete with women for the role of nurturer and caregiver.
I'm sure some might think of me as a neanderthal for my shocking assertion that men and women have fundamentally different skills, interests, and behaviors because of biology, but it seems like common sense to me.