As usual, I will present my contrarian view. While I agree that obsessing about sex (real of fantasy) is unhealthy from both a physical and spiritual standpoint, when Jesus presented that lusting after another in your mind is just as evil as the physical act, He was presenting a standard that most would agree is impossible to meet. True, when we realize what we are doing or have done, the regret and repentance comes into play. But the standard of it never entering my mind outside of the confines of marriage has proven to be a standard too high for me. While I have been 100% faithful to my wife of 35 years, I can't honestly say that lustful thoughts don't ever enter my mind when a beautiful and sexily dressed woman catches my eye. It's impossible to avoid those situations in today's society. Not saying I'm proud of it, just stating my experience throughout my 68 years of life.
Another question concerning this subject is the difference between temptation and sin? When does thought along sexual (or several other ) lines cross over from temptation to sin?
The standard set by Jesus tells me that I will never meet the qualifications through my own strength of will; and I NEED Him as my Lord and Savior; both to save me from my iniquity and to guide me onto righteous paths or I am doomed.