I'm looking at it from the perspective of benefits vs the detrimental aspect of it. Sure, a man, for example, would benefit from a biological standpoint, if he is able to propagate more in regards to his off-spring. A woman, would benefit more from having various sexual partners with strengths that would benefit her offspring and her rearing said offspring. But we also need to weigh that against the costs involved, from a health standpoint, to one that could very well diminish the capacity to care adequately for the offspring (if one is stretched out with children all over the place, the children suffer in a variety of ways), not to mention the fact that children do suffer psychologically when parents have affairs, which can lead to their not desiring to have children or even forming a long term relationship with another person, because of the psychological damage they may have suffered. Are there benefits from a biological point of view? Sure. But there are also costs that could completely override those benefits which would not one's chance of seeing one's genes live on in the family line, if you will. I find the question of whether infidelity should be allowed or not to be somewhat bizarre. From an emotional point of view, humans form deep bonds and we are emotionally tied to relationships for the most part. We are, for the most part, social beings. Once that trust is lost, we find it hard to trust in others in the same way again and if one is of child bearing age, one might not want to commit and risk the pain again. Those choices, however emotional they may be, do have an effect on biology in the long run. It is hard to discount the sociological and psychological aspect of it when it is so intrinsically tied to the risk of those involved, not being able to form bonds or desire to have children in the future. In that regard, it ceases to be a successful biological strategy. Our ancestors formed groups and had very strong bonds within those groups. Their survival depended on it. Cheating within those groups, would break that trust. Trust issues would pose a risk to the family group and the community group itself. Hence, in a way you are correct, our ancestors did not have the modern luxury of trust issues because if they did, they could very well find themselves dying. Ergo, it explains why from an evolutionary standpoint, monogamy became so vital to our ancestor's survival. A female had to be able to trust that her partner was going to be able and willing to care for and protect their offspring. If the male is off diddling all the other females in the group and having multiple children with the other females in the group, that presents a risk to her offspring's chances of survival because he is no longer in a position to focus on her offspring. Sure, it might have survival value in that one is able to father multiple children with different females. But that survival value is negated by the fact that the male would be unable to provide for all his offspring in an adequate manner, not to mention being able to protect all his offspring in a world that was filled with life threatening dangers. Which explains how and why pair bonding became so vital in our evolution and how monogamy played such a huge role. Certainly, there was infidelity. There still is and it is fairly common. But that doesn't mean it was always beneficial. A male, for example would view infidelity in the female even more negatively, because there would always be the risk that he would be expending his energy to care for offspring that was not biologically his.