Terry Gould’s book The Lifestyle has a good section on how sperm competition works and why men react to potential (or actual) infidelity with a surge of testosterone and increased libido. It’s also the case that we humans get a raw deal from nature: we’re partially monogamous. There are multiple indicators, ranging from testes size to female vaginal response with an “illicit” partner to behavioral cues such as mate-guarding and female propensity for forgetfulness (it’s much easier to avoid being found out if she can’t remember the encounter herself).

Open lifestyles explicitly recognize these human traits and attempt to engineer ways around them that are far less damaging than the traditional monogamous-but-cheating-secretly approach, but I suspect nothing can work 100% of the time.