"The man puts his thing inside, and then stuff happens..."
He'd always ask, "What kind of stuff?"
And the adult in question would shrug, then say something vague like:
"Oh, you know, wonderful fooly-cooly stuff. Kissing and hugging,
and then the sperm and the ovum blah blah blah blah special time
of the month blah blah masturbation is perfectly natural...blah blah blah...
sacred and beautiful love between...blah blah. Does that explain things?"