Giants in Genesis 6

I don't buy the fallen angels explanation of Genesis 6.  I see no other Biblical reason to think that spiritual beings can impregnate humans.  In fact, Jesus specifically said that angels don't marry.

If we read "sons of God" as the Godly line of Seth, those who were supposed to be holy (and remember that "sons of God" is a term used of God's people elsewhere in Scripture), that passage still makes perfect sense, and doesn't require any weird demon-human hybrid creatures.

It doesn’t make sense because of the giants that it produced

It really depends on the interpretations of the passage. But angels (or other heavenly hosts) still may be able to impregnate women even though they weren’t married. They could marry too, because they were fallen angels, who probably don’t care about any of God’s will for them. The argument could swing either way.

I have degrees in biology and spiritual beings mating with physical bodies seems not likely. Jesus was the Son of God so there is one notable exception. May be others but I have doubts. I have an answer but nobody would probably like it.

Let us assume that angels and other heavenly beings have similar genetic material (“let us make men in our own image”). The bigger problem would be how a spiritual entity and a physical human can mate. I have an answer for you there . Time to dive into physics!

*Look up M-Theory (string theory) if you want to know more about this theory*

String theory is a theory that attempts to combine all known forces in the universe, such as gravity, electromagnetism, etc, into a single force. It shows how all of the forces are the same force, just different manifestations of it (A such theory is called a Grand Unification Theory). The idea is that all particles, if you could zoom in close enough to look at them, you will see they are just vibrating strings. A certain frequency of the vibration makes a particle. If we zoom out, it just looks like an ordinary particle. One of the requirements of string theory is that there must be 11 dimensions in total. By dimensions, I mean spatial, like x, y, and z. If string theory is true, then that means heaven can literally be right where we are standing, but because we are 3 dimensional creatures, we can’t see, or interact with it. If you are a higher dimensional creature, you have more power than the ones with a lower amount with you. If 2d creatures existed, we could just peel one off of their universe. 

Now that we have that context, how do angels fit into this? It would mean what is spiritual, it exists in a higher dimensional plane. Therefore, they are not made of of wisps, but are real physical entities that will just be out of our reach. To mate, fallen angels will just have to step into our three dimensions. 
Brain overload for you probably!

I don't believe we are in the physical image of God. I believe we are in the spiritual image of God.

I also spent most of my life doing research in laboratories, Brain overload is not a problem. I have worked as a scientist most of my adult life.

What about both?

John 4:24

God IS spirit, but does that mean everything else in heaven is made of pure spirit?

   Well.... Stephen did see heaven open and Jesus standing at the right hand of the Father. Was it right there, or could he see for thousands of light years distance? 
   And was it Elijah or Elisha who prayed for God to open the eyes of his servant so that he could see what was already all around him (the armies of the Lord)?

   I’ve always wondered that, but never heard anyone else mention it.

   And what would that answer be?

God gave them the ability to see in more than  three dimensions. As for Stephen, that might have been the case, but also remember instances were the sky was “torn open”. Space time being ripped like that is a sign of a wormhole, which can only exist with massive amounts of energy and more energy to stabilize it. If humanity colonizes the entire galaxy, we will have enough energy to do that, but so far, only God can make that happen.

I am a theistic evolutionist so that is why a lot of people wouldn't like my answer. Everything in heaven is not spirit in fact the Bible says we will be given a new incorruptible body to house our spirit.

And if heaven can be destroyed (Revelation) then it may be it’s in higher dimensions, or another universe altogether. Or quantum universes might work.

I’ve always believed that the heavens to be replaced with a new heaven and new earth is the firmament  described in the account of the second day  of creation, in other words, the sky.

Jesus was not an exception of a spiritual being mating with a physical being. 
Jesus was placed in Mary’s womb as a fertilised human egg (The beginning of human life) and was born 9 months later.

   Well, your correct in that I don’t care for the doctrine of theistic evolution, but I don’t disagree with your statement in post #54

I agree but some don't want to think of Christ as a test tube baby.

Mary's conception of Jesus was simply a miracle.  God did not have intimate relations with Mary; that would have been a horrible violation of His own commands.  This does not make Jesus a "test tube baby" any more than Adam and Eve were test tube babies; rather, it was a case of God working a miracle.