I think the children are relevant because you've actually used the bits involved, and presumably seen the occasional anatomical diagram whilst doing so (my big purple book of pregnancy certainly had them)
I have heard of men who didn't understand why we couldn't just hold the blood in and let it out in one go, but I think this kid's in it for the dubious fame personally - assuming he's been through the UK educational system anyway - the only people I've heard ask this were rather sheltered, and religiously educated in less rigorous countries.
My DSes know about periods - DS1 calls tampons my blood thingies, and we've discussed where babies come from. DS2 is only just 3 and still refuses to believe I don't have a willy though, so he still has some to learn.