Genesis 6: Sons of God and Daughters of Men

¹When mankind began to multiply on the earth and daughters were born to them, ²the sons of God saw that the daughters of man were beautiful, and they took any they chose as wives for themselves. ³And the LORD said, "My Spirit will not remain with mankind forever, because they are corrupt. Their days will be 120 years." ⁴ The Nephilim were on the earth both in those days and afterwards, when the sons of God came to the daughters of man, who bore children to them. They were the powerful men of old, the famous men.

When I first read this passage, I was confused as to who 'the sons of God' were. Initially, I thought this referred to all men, because aren't we all children of God? And so I thought that God was punishing men for taking as many wives as they wanted. But upon further research, I learned that there is no sure answer.
Some think that the sons of God are actually angels of God. They were tempted by these beautiful daughters of man, so they left God to go to earth and marry these women. The Nephilim are giants that have extraordinary strength, and are the offspring of these relationships.
Some also think that 'sons of God' refers to descendants of Seth, who have sinned because they have married descendants of Cain. The Nephilim in this case aren't giants, just heroes, but may or may not have been the offspring of these marriages.
Others think that 'sons of God' can be translated to 'kings' of 'sons of nobles' and the Nephilim are either princes or great men.

In my opinion, I believe the second one, that the 'sons of God' refers to the descendants of Seth, and I believe that the Nepihilim are the offspring of the mixed marriages. Whichever answer it is, God punished mankind by shortening their lifespans to 120 years.