Good piece, but in truth, sexual exclusivity is only 10 K years old. Prior to patriarchy, which arrived at about the same time as agriculture, there was a lot less emphasis on who the father of any given child was. People lived in small bands of about 25–50 that shared nearly everything. It’s only when you start to have land and other riches to pass along to heirs that paternity becomes important in a new and different kind of way.
These small bands did know where babies came from because they routinely exchanged members so as not to suffer in-breeding. More on all of that below.