I think Harley is making the point that anyone could go down the path where they have fallen in love with someone else and so one thing leads to another and they commit adultery. I certainly have felt capable of it and even talked with my husband about it before we got married. He never thought it would be a problem for him. The result is that I avoided the slightest hint of impropriety and he thought there was nothing wrong with a deepening friendship with another woman. "Pride goeth before the fall."