When we here these two names called we first think of the human aspect of what this is saying but what James is referring this to is the way people are with their Christianity. Being spiritually unfaithful which is just as much a sin as as adultery between two people. You say how do we commit spiritual adultery? By being a friend to everything the world has to offer and still trying to be a Christian. We have to be careful how we allow things of the world to affect our lives. As the scripture says to be a friend of the world is to be an enemy of God. Adultery is one of the worst sins because it affects the body, By being a Christian having the Holy Spirit and living a life lead by the flesh you would be involving the Holy Spirit in sin and that cannot happen. So in essence you would not be a friend or child of God. Either live and walk by the flesh or the spirit, if you are in the flesh the flesh rules and you are not of God. I pray you will be a Bible believing and truth teaching church today and let God fill your heart with His love and mercy.