In my prvious post, I pointed out that human beings are not naturally born to be sexually monogamous, like the diplozoon paradoxum flatworm that is found on the gills of certain fish. The diplozoon flatworms are obligated to monogamy because in adolescence, two juvenile "diporpa" attach their suckers onto one another and they stay attached, mated, until death never parts them. If somebody wanted to make wedding gifts celebrating complete monogamy, by all means, it should be these funny little flatworms!
OK, so if monogamy is not natural, what about adultery? Here we have more data. According to genetic testing laboratories, approximately 10% more or less of children taken in for random genetic testing are not the biological offspring of their "fathers". This is called "paternal discrepancy" and it is widespread. This finding supports the old axiom, Mommy's babies, Daddy's maybes. Until DNA testing, men could not really know their own children, and were often unpleasantly surprised.
The outcome of infidelity, or adultery, can be dire indeed. It is one of the major causes of murder throughout the world. Betrayed people kill their spouses, the lovers of their spouses, their children, and themselves with some regularity. Even if adultery does not result in murder, it can result in enormous psychological damages, including severe depression, anxiety, and PTSD.
So - monogamy may be difficult, and there may be good reasons to change partners as the years march on. But infidelity is a high risk, dangerous way to go about it. Far better to declare an old relationship just that, old, or out of date, then break up and move on, than try to "sneak one in on the side", running the risk of the kind of sad dramas of Anna Karenina, Madam Bovary, Hester Prynne, and even Molly Bloom. Those ladies are all fictional. Think of John Edwards, Prince Charles, Tiger Woods, Princess Diana, and Elizabeth Taylor. We are fascinated by adultery because it makes for high drama, but when it comes to your bedroom, you should have a good counselor and a good lawyer in mind.