Taxi Driver - S8-E19
Continuity mistake: When Crowley comes to the ship and finds Kevin Tran he blew out the glass on the ship to get rid of the sigils. When Sam and Dean get to the ship to check on Kevin, and they notice he's gone, all the glass isn't broken and the sigils are still there.
Answer: Angels themselves may not have genitalia, but their vessels do. Nephilim occur when an angel in a vessel reproduces.
Greg Dwyer