Problem with monogamy is that it goes against human nature. From evolutionary perspective, our ancestors gained a lot by cheating. They passed on more and diverse genes into future generations. So part of us are those evolutionary program breeds that lead us to want to cheat.
The other problem is that we're living a lot longer than before. So now a couple that gets married at 20 is expected to be monogamous for 60 years. That's a long time to be with one partner. It is simply put like that. If you're told that no matter how like, much you like a particular restaurant that you must eat at this restaurant everyday for the rest of your life, I think you 're gonna wonder what it's like to eat somewhere else.
So obviously monogamy puts a very heavy burden on a couple and therefore, it is important for a couple to continually negotiate the terms of monogamy. I think there is nothing wrong to renegotiate these terms from time to time. Perhaps to allow for some transgressions, perhaps allowing for talking to someone on the internet or watching pornography or sexting whatever it is that makes that partner comfortable and it's something that each couple needs to renegotiate every decade or so.