Continuity mistake: When Dean reads about the Roosevelt Asylum in his dad's journal, there is a picture of a Wendigo on the opposite page of the newspaper clipping but in a different scene, Dean checks the article again for information on the South Wing in the Asylum but this time on the opposite page of the newspaper clipping, there is no picture.
Continuity mistake: Season 1 episode 2, Wendigo conflicts with this episode. Originally the wendigo information is printed on the left-hand side of the journal. On the right side is loads of symbols and text. Here the "same" page featuring the wendigo can be seen when Dean and Sam are searching for the asylum information but it's different.
Answer: Angels themselves may not have genitalia, but their vessels do. Nephilim occur when an angel in a vessel reproduces.
Greg Dwyer