Here's the answer:
Don't feel bad. Nobody explained it to me, either, and I must say I made quite an impression that first night with the honey and feathers. But now I'm hip. The significance of the birds and bees isn't what they do, it's simply that they do it, "it," naturally, being a tussle in the tumbleweeds, or wherever it is that the lower orders engage in sex. As such it's the perfect euphemism for a culture so prudish that even publishers of girlie magazines used to airbrush out the pubic hair.
Where exactly "the birds and the bees" originated nobody knows, but word sleuths William and Mary Morris hint that it may have been inspired by words like these from the poet Samuel Coleridge: "All nature seems at work ... The bees are stirring--birds are on the wing ... and I the while, the sole unbusy thing, not honey make, nor pair, nor build, nor sing." Making honey, pairing ... yes, we can definitely tell what Sam had on his mind.
The Morrises offer the theory that schools in years past taught about sex by "telling how birds do it and how bees do it and trusting that the youngsters would get the message by indirection." Right. Luckily for the perpetuation of the species, there's always been Louie in the schoolyard to explain how things really worked.