The most likely interpretation is that these "sons of God" were angels:
"'Son of God' in the OT can refer to a 'heavenly court' of supernatural beings [angels] (Gen. 6:2, 4; Job 1:6; 2:1; 38:7; Ps. 29:1; 82:6; 89:6 ...)" - p. 571, Vol. 4, The International Standard Bible Encyclopedia, Eerdmans Publishing Co., 1988. [Bracketed information added.]
Further supporting this is how the Bible mentions that it was disobedient angels that left their place in heaven during the time of Noah, came down to the earth, and took on fleshly bodies:
"The angels that did not keep their original position but forsook their own proper dwelling place..." (Jude 6)
Their reason for doing this is explained in the following Scripture:
“The sons of the true God began to notice the daughters of men, that they were good-looking; and they went taking wives for themselves, namely, all whom they chose.” (Genesis 6:2)
Supporting the conclusion that these were ANGELIC "sons of God" is the apostle Peter’s references to “the spirits in prison, who had once been disobedient when the patience of God was waiting in Noah’s days” (1 Pet. 3:19, 20), and to “the angels that sinned,” mentioned in connection with the “ancient world” of Noah’s time. (2 Pet. 2:4, 5)
For more, see these posts at my other blogs:
Were the "sons of God" mentioned in Genesis 6:2 angels, and if so, how could they produce offspring (Nephilim) in the time of Noah?
http://searchforbibletruths.blogspot.com/2010/06/since-angels-are-spirits-how-did-they.html
Identifying the Nephilim
http://defendingjehovahswitnesses.blogspot.com/2013/10/identifying-nephilim.html
"'Son of God' in the OT can refer to a 'heavenly court' of supernatural beings [angels] (Gen. 6:2, 4; Job 1:6; 2:1; 38:7; Ps. 29:1; 82:6; 89:6 ...)" - p. 571, Vol. 4, The International Standard Bible Encyclopedia, Eerdmans Publishing Co., 1988. [Bracketed information added.]
Further supporting this is how the Bible mentions that it was disobedient angels that left their place in heaven during the time of Noah, came down to the earth, and took on fleshly bodies:
"The angels that did not keep their original position but forsook their own proper dwelling place..." (Jude 6)
Their reason for doing this is explained in the following Scripture:
“The sons of the true God began to notice the daughters of men, that they were good-looking; and they went taking wives for themselves, namely, all whom they chose.” (Genesis 6:2)
Supporting the conclusion that these were ANGELIC "sons of God" is the apostle Peter’s references to “the spirits in prison, who had once been disobedient when the patience of God was waiting in Noah’s days” (1 Pet. 3:19, 20), and to “the angels that sinned,” mentioned in connection with the “ancient world” of Noah’s time. (2 Pet. 2:4, 5)
For more, see these posts at my other blogs:
Were the "sons of God" mentioned in Genesis 6:2 angels, and if so, how could they produce offspring (Nephilim) in the time of Noah?
http://searchforbibletruths.blogspot.com/2010/06/since-angels-are-spirits-how-did-they.html
Identifying the Nephilim
http://defendingjehovahswitnesses.blogspot.com/2013/10/identifying-nephilim.html
