And I bet you're saying "Whaaat?!"
When I was in Physics 1 as a freshman engineering student at the University of Minnesota, one of the topics of study was falling-body problems. (No pun intended.) An object dropped from a height will cover distance downward according to the formula
s = (1/2 * g * t^2) where g is the acceleration due to gravity (and * means times, and t^2 means t-squared)
and the velocity of the object will be
v = g*t
To make things more interesting (read: complicated), the downward acceleration is independent of any forward motion the object might have, such as if the object were dropped from a moving airplane. Thus both downward and forward motions need to be analyzed. The math will show that a falling body in motion when dropped will follow a curve called a parabola, and its point of impact can be easily calculated.
But it's not as easy as that. There is air resistance, which increases proportionally to the mass and shape of the falling body, and (to make things even more interesting) changes the path of the falling body from a parabola to a cycloid.
So I asked the professor, "When do we get to figure air resistance?" and he replied, "Come back when you've had about five quarters of calculus."
Translated, this means: "You don't have the need or ability to understand that yet."
So with sex education. Little kids don't need to know about the "mechanics" of sex, because they don't have the need or ability to understand it (and though I'm not a parent, I would bet all my blog dollars they don't care either.)
I would venture to say that almost all little kids, when asking why Mommy's belly is getting big, being told that their little brother or sister is growing in there, will ask "Well, how'd he or she get there?" Parents correct me on this one, but I bet it will be quite enough to tell a small child, "God put him / her there."
Parents can tell their kids the equivalent of what the physics prof told me.
And I cannot state strongly enough that it's nobody's business but the parents' to give the child any more information, because the parents know their child better than any teacher possibly could, and they know when the child is able to absorb and assimilate the information.
(This applies only to ordinary kids in ordinary families, of course. The "special cases" are something else entirely.)