If I understand this correctly, Michael, you apply the ambivalence of
ANWQEN only to PNEUMATOS. But, in my view, it should apply to the whole
phrase, EX hUDATOS KAI PNEUMATOS. (That's what at least the parallelism
between vv. 3 and 5, as well as the use of the preposition EK, imply).
If water-birth were to stand for natural birth, as you are inclined to
believe, it could neither mean "another birth" (as Nicodemus wrongly
took it) nor "birth from above" (as Jesus probably meant it).
In the other interpretation, which I would find more defensible, Jesus
speaks of _one_ birth from _two_ elements. Carson, I think, compares
this expression (with two anarthrous nouns governed by the same
preposition) to DI' hUDATOS KAI hAIMATOS in 1 John 5.6 -- the latter
referring to _one_ coming via _two_ distinct events.
Adrian Popa