Birds + Bees = ???

Look, I need some help.

Was browsing Comixed, found this:


and it hit me: where did this expression: "the birs and the bees" come from, anyway?
Who thought of it?
And what's the connection anyways?
The only thing that seems obvious is that they both fly..
But why birds and why bees?

Aah, the never answered question..
So please, if you have any theories don't hesitate to cantact me.

Anyway, in my search for the answer I found some pretty awesome comics:


and my favourite:


If I think about it..