The problem of sex (or why I'm not an atheist)
it doesn't quite explain how very complex organisms developed fully distinct yet necessarily complementary reproductive apparel
That is the really easy bit. I'm sure a bit of research will turn up a lot of info and extant examples of external fertilisation ----> internal gestation. Just last week I read an article on the evolution of the placenta.
The origin of sex was the real hurdle and I have explained that in some detail above.