He's a good kid and he's always known that he can ask me anything and I will always take him seriously. So it wasn't even awkward.
He said he'd been curious what "the sex" is, because apparently a couple of fifth graders got in really big trouble for "doing the sex" at his elementary school this year. (This is of course the first time I heard about it.) Who knows whether it really happened or not, but as I said -- best to get out ahead in front of it.