It is the most unnatural thing for one man to be with one woman.
It is conversely the most unnatural thing for one woman to be with one man.
In our basic natures sex with more than one partner is the norm.
That being said. As a married Christian man. Husband to one wife and father to our four children.
God has also put within us the ability to transcend our nature our flesh as it were.
To attain a measure of divinity and power over this flesh through Jesus Christ.
I didn't get married because I thought it was going to be some crazy sex party and my wife was going to fulfill my porn fantasies.
I got married as a covenant with my wife before God for the express purpose of producing children and raising them in love; in the manner written on my heart by Jesus Christ.
God has so deepened my love for my wife far beyond any filial or erotic love that one could imagine.
Usually people who are unfaithful in marriage are thinking of themselves. Usually it is just self centerdness.