Monogamy is a life style choice!
Humans were given "free will".
It's almost like asking if humans were meant to be vegetarians.
We have the power to live our life as we see fit.
If you don't want to have sex outside of your relationship you won't. If you do want to have sex outside of it you will. Nothing and no one forces you to do something you don't want to do.
The key to life is knowing yourself. If you know deep down that you could never spend the rest of your life having sex with just one person then you should not make a promise to do so. Life is a personal journey and not everyone has the same desires nor the capacity to live a particular life style choice. We have to be honest with ourselves before we make promises.