Marriage is designed to be an exclusive emotional relationship between husband and wife; each spouse has the responsibility to follow through on their commitment to meet the other spouse’s emotional needs. Marriage is a leap, and exclusivity means that you are giving up your fallback position; you are putting all of emotional your eggs in one basket.
Jesus attributed the Biblical regulation concerning divorce the hardness of the heart of man rather than to the will of God, and referred the Pharisees back to the original intention of God, from Genesis chapter 2, that being, that a husband and a wife are no longer two separate individuals, but are joined into being one by the creative action of God.