(in) single file phrase
Used to refer to a line of people moving or standing in one line, one behind another.
The soldiers were walking in single file to the hall.
We have to follow each other in single file in the school canteen.
Attention! Please line up in single file to our first destination.
The word "fine" has had the sense "row of persons or things one behind another" since the 1590s. It is originally military, from the French verb in the sense of "march in file."
People tend to like forbidden or illegal things just because they are forbidden or illegal.
He must have believed that stolen fruit is the sweetest, and he, therefore, cheated on his partner.