I believe that the “porn culture”, rampant marital infidelity, and sexual promiscuity of modern times is a good example of the sort of immorality that Christ was speaking against as “lustful” during his ministry.
Unfortunately there seems to be a large number of threads on here posted by good men who are worried that they are sinning because they have sexual thoughts when they go to the beach, see a provocative commercial or an attractive female walking down the street.
It is sad that in this day and age something so natural and beautiful can be make a man become self-loathing, riddled with guilt and shame because some of the early church fathers (like Augustine) couldn’t control themselves during their youth.
When Christ spoke of lust and “adultry in the heart” I don’t think he was talking about normal sexual thoughts and feelings experienced by man and woman who haven’t yet married. It is these very sexual thoughts that initiate a man’s interest in a woman as a potential wife in the first place. Without those initial sexual thoughts, there is almost no chance for marital type interest to even get off the ground.
How great would it be if we all stepped back and re-examined how we define the word “lust”. While modern culture is learly out of control, it’s just as rediculous to constantly run around in a fog of shame.