Half of the kids in these classes might not have heard anything though. Bringing in sex ed at such a young age then brings it to everyone and I don't think that's appropriate at such an age.
See here...
Also parents would be able to have a choice of what their children learn. Where I went to school the parents were invited to preview the sex ed. classes to see if that's what they wanted their child to learn. If not, parents were allowed to allow their children to go to the library instead of participate in the class. I believe what Obama was talking about would be something similar.
I just don't agree with the mentality of "well they'll hear it anyway so let's just let them know now."
That may be where the disagreement stems from.