Continuity mistake: Gary (disguised as Sam) sits at the bar and orders a banana daiquiri. The woman next to him tries to engage him in a conversation as he sips his drink, and each time the camera switches between him and the woman, the level of his drink changes noticeably up and down.
Other mistake: When Dean is listening to Sam's voice mails that he left while in Gary's body, the voice in the recordings are of Sam. If Sam was in Gary's body, they would have been of Gary's voice.
Answer: Angels themselves may not have genitalia, but their vessels do. Nephilim occur when an angel in a vessel reproduces.
Greg Dwyer