The Hebrew term 'son of God' used here is found in other places in the Bible meaning different things. In some places it means angels, in other places it refers to various followers of God, and in the NT, the equivalent term refers to Jesus.

I would say that within the context of this verse, it is probably not referring to angels or Jesus. Thus, it is probably simply referring to godly men. "Daughters of men" on the other hand is probably referring to ungodly women. It could even specifically be referring to the people descended from Seth and the people descended from Cain (written about in the previous chapters).

So basically the most likely interpretation of this verse is that godly men were marrying ungodly women.