Anyway, my point is that skin-to-skin contact can lead to the release of oxytocin in both males and females and this can facilitate falling in love and pair bonding. Pair bonding is rare in primates, and doesn’t happen when apes live in groups. It is usually prevented by the dominant male who will try to monopolize all the females.
Stone knives would have had a leveling influence, undermining the rule of the strongest male. They would also have led to social disruption, because now there would be a continual free fight over women. Previously the dominant male would have controlled this problem, but stone knives may have eliminated his role.
Two million years ago agreements were not about peace, order, and good government. The agreement had to be simple, it had to be comprehensive, with no exceptions, and it probably had to do with access to females. Our ancestors had the right hormones to facilitate pair-bonding, but they didn’t have the right social systems until the invention of stone knives forced their hands.
We all can and do feel judgmental about people who have affairs. We realize that they are doing it out of powerful desires, but judge these people for not constraining their desires. The fact is that if people didn't actively constrain themselves, monogamy would be a joke. The only thing natural about monogamy is that it reflects the pair-bond, the deep mutual attachment that can form when two people fall in love. But the trouble is that, in many cases, love doesn't last, and it can be overridden by new attractions. That's why the group had to come together and make a collective commitment, simultaneously creating the social institution of monogamy and the first moral system. My guess is that initially it was simply an agreement to control violence, and allow for pair-bonding and social stability, and the initiators had no idea of the positive consequences that would ensue.