As a spouse in a marriage in a religious family, I can say I have never based my fidelity on some fear of punishment by God. In fact, fear of punishment by God rarely enters into the equation about what I do and not do. Just like with non-religious couples, fidelity is about commitment and moral values; it is about respect for the marriage and the spouse; it is about strong family values. Those are all Biblical principles (as well as just common moral principles) and really have nothing to do with fear.