Thinking about it though, he wasn't the only one. Besides, Gabriel was one of the archangels assigned with destroying the original Nephilim, that was what the great flood was about, so I don't see him taking the chance..
Didn't Anna mention that she was stationed far away from home, and how lonely that felt, way back in s4?
Now think about it, Anna had already torn off her wings, had fallen, had become human, yet still to heaven that wasn't punishment enough for the horrible crime she'd committed.
But her crime was to fall in love, to want to love and have sex like a human.
So what if she did so, what if she fell in love and had a man's baby, and gave birth to it, to a Nephilim. The one crime pre-Lucifer that got angels banished to earth/hell.
What if that woman was Anna's daughter?
What if Anna's crime was giving birth to a half human, half angel hybrid, an abomination that shouldn't exist... and heaven never forgave her for it.