Well, I don’t know about fallen angels, if they were still angels, or if they took on our form, but they were able to breed. Not sure the bible is specific on how this was accomplished- whtehr they remained angels or not
The mechanics(science) of many things in the bible are not detailed or explained..
As in spiritual possession.. What being possessed?..
What is "spirit/Spirit"?.. i.e. God, Angels, US, demons...
What "leaves" and "remains" when a human body dies?..
Gender seems so arbitrary.. incomplete.. primitive.. to me..
Could just be that gender is a second reality..
-OR- that gender speaks of something greater..
As an archetype(model) of something in us all..
Meaning; Maybe the female and male side of us all is spiritual reality..
That we're all greater than we seem.. all of us.. in many aspects..
EVEN (but not limited to) gender..