Elohim had blessed the human beings He had created and allowed them to have children. That way A'dham became the father "of a son" in his own likeness, "after" his own image. We were not told this about Cain and Abel simply because the text did not wish to discuss them at length. In this chapter 5 Moses goes straight to the third son of Adam, Enosh. It is noted that from that time the people started to call upon the Name of Jehovah God.
The genealogy of Adam gives us a picture how from those being created in the image of God, were born new creatures. Male and female were created by the Only One God, Who blessed them and gave them the possibility to bring forth new blood, so that the purpose of God, to fill the world could be fulfilled.