And Then There Were None - S6-E16
Revealing mistake: Samuel is very much dead, laid out on the table in the canning factory employee lounge, but as Sam stands over him, talking about whether or not it was right to gank him, you can see Samuel's chest rising and falling as he breathes. (00:26:30)
Answer: Angels themselves may not have genitalia, but their vessels do. Nephilim occur when an angel in a vessel reproduces.
Greg Dwyer