Sons of the first creation. Note that when Genesis describes the creation of mankind, it has two different stories.
In the first one, they are 'male and female'... there is no story about how first it was male, and then a female was taken out of it.
In the first one they are given dominion over the whole earth and given all the plants to eat. There is no passage about being put in Eden specifically and then God letting them eat from all kinds of trees except one. That is only in the second story.
Again, in the first one there is a chronology about how everything is formed, from the plants, to the birds and sea creatures, to the land animals, and then to man. But in the second one, Man is first, then he is placed in the garden where God causes plants spring up, and -then- God forms animals and brings them to be named.
So it would seem these are two separate stories about separate kinds of beings. The first kind is made in God's image and not put in a garden with any restrictions; the second is made from the dust of the earth, God's breath of life is breathed in, and a very detailed story is given.
And there you have an answer for the sons of God -- they're from the first creation in Genesis 1. They are not angels, but they clearly aren't quite like humans, if they can conceive giants with humans.
I don't know if this is completely true, but it works better than just saying they were angels (are angels sons or servants? are angels in the image of God?), and also better than saying giants are a result of human interracial marriage. There's strange creatures in Revelations, so it is not as if angels and man are the only spiritual beings God created.
You don't have to take my word for it. Some things we only know when God wants us to.