There is a logic here that only comes out when one exegetes the Hebrew words themselves…let me explain…
Nowhere in Genesis, or anywhere else in the Bible for that matter, are the sons of Seth referred to as the sons of God. Likewise, nowhere in the Bible are the “daughters of men” specifically a reference to the daughters of Cain. These could well have been any women including the daughters of the Sethites. In almost every other place where this specific phrase bene ha'Elohim (sons of God) is used in the Old Testament, it is clearly speaking of angels (Job 1:6, 2:1, 38:7; Psalm 89:6, 103:20; Daniel 3:25).
Yet another problem appears to be in the words themselves that are used. The first phrase is bene ha’Elohim and the second is literally daughters of Adam. From this it would appear, if this interpretation is correct, then Seth himself would be Elohim, and Cain would be Adam. But weren’t the daughters of Seth also the daughters of Adam? Of course they were. And was not Seth made in the image of Adam (Genesis 5:3) and not God? If Seth’s daughters (the daughters of God) were not included why were they destroyed as well? Why were they not more properly included as the daughters of Elohim (if the Sethite theory is correct)? If being daughters of Elohim they were not affected by this marriage corruption plot, then why would they also have to die in the flood? If the daughters of Seth were also among “daughters of men” then this makes sense, but if this refers to the daughters of Cain only it does not, otherwise the phrase excludes them and they should have been spared.