"Ye have heard that it was said by them of old time, Thou shalt not commit adultery; but I say unto you, that whosoever looketh on a woman to lust after her hath committed adultery with her already.
Has done it. What?
Has done it. What?
All he did was to look and think. That is all. 
But he has committed adultery, so that in forbidding to commit adultery, He forbids a look or a thought which, if followed up, could possibly lead to it.
The law of God is intended to control the actions,
by controlling the very spring of the thoughts.
That is the principle upon which the Bible deals with mankind."
A.T. Jones/Matthew 5:27,28