Because, as stated in the manga, the Belldandy we see on earth is not her true form... it is just a 3 dimensional recreation of her 10 dimensional form. Furthermore, it is constantly being regenerated at a subatomic level (she can even stop this regeneration temporarily, which causes her to fade out). It has also been established that earth medicine does not work on her, and booze doesn't effect her (and alcohol is involved in the begining of lots of pregnancies).
It was also somewhat established that goddesses don't have, um, the monthlies. Which is lucky for K1... women living together tend to get their cycles in synch... with all of them having goddess powers, PMS would definitely be a killer!
Logically though, they can't have children. I doubt K1 has realized that yet, and it may be one of the reasons why Bell looks sad once in a while. I can see her wanting to bear a child for him.
If Fujishima-sama does go into it, it will probably be one of the most heartrending stories ever told in a manga.
Larry
19-Feb-2005