Dear Kripke: Angels v. Demons & Supernatural canon
Azazel is one of the original fallen angels, tossed out by none other than Raphael.
While inside John, Azazel revealed that Meg and her brother were his children.
In the bible, the offspring of an angel and a human was a giant.
If Azazel took a human wife, his children wouldn't have been human, they would have been giants.
If Azazel took a demon for a wife, his children wouldn't have been human, they would have been demons.
If Azazel took an angel for his wife (which I don't think is possible) their children would have been some combination of demon and angel and certainly not human.
Ruby said that humans can eventually become demons, as she was one.
In Houses of the Holy, we are told that angels are angels only and humans might go to heaven, but they don't become angels.
In Meg's most recent appearance (in the child bride of Macauley Culkin) she mentions the demons are dreaming more than they ever did as humans. If Meg is the offspring of a Demon/Fallen Angel, she would either be a giant, or a demon, or angel/demon mix, but never a human. Why would she reference anything about herself being human?
Angels can only occupy a human's body if they have permission.
Lucifer needs permission to occupy a human's body, because while he's a demon, he is a fallen angel, so the rule still applies.
If Azazel is also a fallen angel, wouldn't he need permission to enter a human too?
And if so, how the hell did he get the green light to possess John Winchester?