David Taylor
Puritan Board Freshman
You don't have to make him out to be a dimwit for the physical interpretation. It perfectly aligns with the flow of the conversation.But (assuming that Nicodemus is a smart guy) the most reasonable interpretation of Nicodemus' comments in verse 4 is that he means something like, "I can tell you can't be talking about physical birth, so please explain the spiritual truth you must have in mind." And Jesus responds with comments about spiritual birth. It makes everything fit together much better.
The alternative is that you must believe Nicodemus is a complete dimwit. Admittedly, this passage gets taught that way sometimes, because it's fun to imagine Nic is clueless. But we can and should do better.