Matthew 22:30
For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven.
This is about as close as it can come to saying angels do not marry so I would say the sons of God are not angels.
It could be referring to Seth and Cain although I see no reason it would produce giants.
I have not changed my mind, I still believe God created man
on the sixth day which was not literal but a time period.
And created Adam at a later date, who`s descendants were called the sons of God.
And it is obvious in order to believe this I can not believe in the young earth theory which I would not be inclined to believe in anyway.
"Nephilim" does not necessarily refer to giants. According to Wikipedia (I know, it's not an authoritative source):
The Brown-Driver-Briggs Lexicon gives the meaning of Nephilim as "giants".[1] Many suggested interpretations are based on the assumption that the word is a derivative of Hebrew verbal root n-ph-l "fall". Robert Baker Girdlestone [2] argued the word comes from the Hiphil causative stem, implying that the Nephilim are to be perceived as "those that cause others to fall down". Adam Clarke took it as a perfect participle, "fallen", "apostates". Ronald Hendel states that it is a passive form "ones who have fallen", equivalent grammatically to paqid "one who is appointed" (i.e., overseer), asir, "one who is bound", (i.e., prisoner) etc.[3][4] According to the Brown-Driver-Briggs Lexicon, the basic etymology of the word Nephilim is "dub[ious]", and various suggested interpretations are "all very precarious".[5]
The majority of ancient biblical versions, including the Septuagint, Theodotion, Latin Vulgate, Samaritan Targum, Targum Onkelos and Targum Neofiti, interpret the word to mean "giants".[6] Symmachus translates it as "the violent ones"[7][8][9] and Aquila's translation has been interpreted to mean either "the fallen ones"[7] or "the ones falling [upon their enemies]".[9][10]
So, instead of giants, the nephilim could merely be "fallen ones", "violent ones", etc, rather than "giants" I do not think there is a definitive answer to this. I think some of the confusion comes from translations that are a tad too definite in their choice of words, using "there were giants in those days" as the KJV does, rather than the vague word Nephilim. I still think R.C.'s explanation makes sense, and it is what I believe; but there is not a lot to go on in the small passage we have to work with.