I'm not going to debate whether sex-ed should be taught in the schools, but I learned where babies came from when I was 5 years old, and it didn't do me any harm. It was in a book called, "A Doctor Talks to 5 to 8 Year Olds", and basically covered pregnancy from conception to birth. Later, I learned how conception was achieved, I think when I was about 7-8. And around that same time I remember a boy playmate who was a couple of years younger than me, telling me what he learned about babies, and he had the info pretty much correct, and sounded like he was reciting from an encyclopedia. I don't think you'd be doing your sons any harm whatsoever by telling them where babies come from and how they're made. They can handle the information without becoming 'sexualized'.
Halls isn't doing them any harm by keeping it close to the vest for now, either. I'm not aware of any studies showing kids are screwed up if they don't hear this stuff by X age. I learned that stuff even earlier than wimpycat did, but countless people have been perfectly well adjusted learning it much later, even waiting until the threshhold of puberty.