The problem with Christian ethics is the word "sin."  Look at a person cross eyed and you have sinned no less than if you committed murder.  Look at a woman with lust and the man has sinned just as much as if he had raped her.

There is no sense of proportion in Christian ethics.

A rational ethical system recognizes that almost all if not all acts have good and bad consequences, some known to the person doing it and some not known.  There are therefore degrees -- and there are pluses and minuses to be considered.

Looking at someone and feeling lust can be very pleasurable, a plus (pleasure is a good thing) and, if it is kept private, should harm no one.  The words Jesus is supposed to have said, therefore, are just plain wrong and probably harmful if they generate guilt or fear.

Christian ethics seems to have come from a bunch of extremely judgmental, sex fearing men who almost certainly had their own personal problems.  Jesus said not to judge, but Christians are great at it.