She did go to schools with comprehensive sex ed. (OK, maybe not comprehensive in the sense we think of it as being with lessons about homosexuality, but to put it mildly, that's not the relevant issue.)
My guess is that the explanation, to the extent that there can be one, is:
1. Teenage kids can be very stupid when the hormones take over.
2. There's not a lot to do in rural Alaska.
Though I remain mildly curious on the whole "let's fly back to Alaska while I'm in labor" issue, I'd be OK if we could all stop talking about the Palin family's vaginas about now.
Gotta say, Obama's being a real class act about this, between his public threat to fire any of his staffers who push the story and his bringing up that his own mother was 18 when she got pregnant with him.
Let's follow his example.