In Genesis 6:2, it says, "the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose." I thought that the only Son of God was Jesus. Does this mean that Jesus has brothers, or that it was Jesus, or did it have anything to do with the Trinity being three personalities? Also, why is sons not capitalized? And if they weren't human, why did God decide to wipe out the humans because of them?
I've read two basic answers.
The sons of God were fallen angels (demons) that impregnated women, creating a race of giants. These nephilim are what caused God to finally decide to wipe out an already wicked race* and start over with Noah** and his family.
The sons of God were merely the righteous sons of Seth who intermarried with the (presumably pagan by this time) daughters of Cain. Why this union should produce such noteworthy men (v. 4), and should evidently anger God so, is not clear.
There are no doubt other possible answers but these are the primary ones I'm familiar with.
*v. 7 suggests that ALL flesh the world over - human and animal alike - may have somehow been corrupted, but this is speculative.
**Interestingly, Noah is distinguished as having been "pure in his generations" (v. 9).