The fact that Genesis 4 opens up with “Adam KNEW his wife” is striking. Yes, this is the Hebrew way of saying sex, but the wording suggests that immediately prior to this chapter, Adam did not know Eve. Not really.
We read that “their eyes were open” in Genesis 3, but perhaps “seeing” does not mean “knowing.” It requires something more.
Now Adam knew Eve his wife, and she conceived and bore Cain, and said, “I have acquired a man from the Lord.”
Genesis 4:1 (NKJV)