Spiritual seed sure....not physical....because neither Satan or any angel is a physical being...they are spirit beings.
Spiritual beings that, throughout Scripture, do very physical things, themselves, and have very physical capabilities, themselves, without possessing human beings.
What would it be about a spiritual being possessing a physical being, causing him to act sexually, that would genetically change the human seed to produce giants? (Genesis 6 ; and the other breakouts of nephilim throughout the OT, " . . . and also AFTER that . . . " i.e., after the days of the flood. ---the giants in the land of Canaan seen by the twelve spies ; the giants in Saul and David's day, etc.)
Does possession of humans by spiritual beings change genetic structure?
Do not confuse the meaning of physical with our worldly concepts.
Angels, while identifiable with spirit beings, also are associated with having bodies. Some fallen angels have lost their bodies, but still exist spiritually.
Job 2:2 refers to Satan walking to and fro the earth.
It is true that some demons do not have bodies as we know them, but they are still able to engage in actions which result impact physical phenomenon. They can interact physically, throw a man’s body around, engage in a fistocuffs, although not be visible nor touchable by the man. They are in a different domain, which we really do not need to know at present as His Plan for us doesn’t require it.
While many may not believe in the Gen6 basis for Gap theory, IMHO, it is wise not to label it as false, but instead simply accept His Word as it is.
If He had wanted us to know more about it, He has the ability to communicate it, and does if the situation so warrants it. IMHO they are not situations to be coveted nor sought, as that tends to be the domain of witchcraft, which is highly forbidden.