Hi there,
I don't wan't this to turn into religious / non-religious discussion, so please be concrete in your answers.

I was raised in a religious family but now in my mid 30's I'm not religious anymore.
Looking at the couples that base their faithfulness to one another on a fear of a God keeping away from infidelity I often find as a pressure or a control.
So many times I feel that this fear isn't enough to keep their faithfulness towards one another.

I wonder, in a relationship where couples do not rely on this religious fear to keep their relationship pure without infidelity, on what is their faithfulness to one another based?
Is it weaker or stronger than the religious one?
What are the principles of keeping infidelity away?
Thanks