You are completely leaving out the emotional component.
If there would be no emotional component, there would be no further procreation because it would be too much effort.
But exactly that emotional component causes things like this.
It turns out it is very easy for people to get attached to other people, if those people manipulate them well.
Which is again, not very difficult, but most people have their conscience to stop them.
Some people either do not have a conscience or do, but do not care.
Maybe for those people there is a certain pleasure in fooling two people, and getting away with it.
Maybe more than two people.
But of course, this is not imaginable if one has not experienced it.
Emotions, being abstract concepts, are not like physical things.
They cannot truly be conveyed in any physical medium.