Nature has played a cruel trick on our species. We’re semi-monogamous. There are various “tells” such as the size of the male testes. In primates that are promiscuous (like Bonobos) the testes are huge relative to overall body mass whereas in Gorillas where a dominant male enjoys a harem the testes are tiny relative to overall body mass. Guess what human testes say about us? Yup, “semi monogamous.”
That’s why we have all those mate-guarding behaviors hardwired into us, like jealousy and suspicion. It’s why men ejaculate more semen with more sperm after they’ve been separated from their female partner for a while (it’s called sperm competition). It’s why women often have a surreptitious little adventure around ovulation time, and then often genuinely forget all about it afterward (studies indicate about 10% of children haven’t been sired by the male who believes he’s the father).
The happiest and most sane people I know are those who accept and embrace this unfortunate fact of life and recognize that it’s rarely infidelity that wrecks a relationship — it’s the lies that are told in an attempt to hide the infidelity. By being open and honest and working through the inevitable jealousies and insecurities, such couples reach a level of mutual trust and intimacy and love that ordinary couples never get close to, or even imagine possible.