One of the major themes of Genesis is the tracing of the line of faith, or the covenant people of God (called the sons of God) and contrasting it with the line of nations (or the sons of men). The succession of God's covenant was (and remains) primarily through families throughout the Scriptures, but particularly so in the Old Testament, and very pointedly in the book of Genesis. The passage is clearly about intermarriage between the covenant people (descendants of Abel) and those outside the covenant (descendants of Cain). That's why these "bastard" children were noteworthy. To the mind of both peoples in those days, intermarriage was considered taboo.