Despite the strangeness of the conversation, when Castiel spoke, he had Ben's undivided attention, as it wasn't just about the one question, it was about many questions that all led to something he was trying to sort out. While Ben certainly had an understanding of topics that likely weren't fit for public discussion, that understanding rarely stopped him when he wanted to know the answer to a question or just wanted to say what was on his mind.
"Yeah, sure. What about stuff like polygamy and threesomes? Dudes in the Bible had lots of wives, right?"
Part of the freedom with which he spoke was the fact that he wasn't yet ten, so while he openly talked about things better saved for non-public discussion, like sex (and being frustrated when people lied about things in order to dumb them down for him or make them 'kid-friendly'), it wasn't with a complete knowledge of said topics like sex. Generally, however, adults didn't stop to think about that, they just heard him talk about sex and assumed he'd somehow figured out all the details.