Person: "How many people have you had intercourse with?"
Me: "Nun."
Person: "How many people have you had intercourse with?"
Me: "Nun."
Listen up, nimrod! The joke is that the person answering the question says "nun" which sounds like "none." So they're playing on words, implying they haven't had any intercourse. It's funny because it's a homophone, and you probably don't know what that is because you're dumb.