Lusting in the heart is one thing. Acting it out is entirely another.
And here’s an interesting truth about lusting in the heart, or thinking about any object of desire (or fear, or any thought which can lead to unhealthy or unwanted actions).
If thoughts are “entertained”, they’ll stick around, make house, and prod people to action.
Thoughts that are wrong can be shown the door, there are many ways to do this. “Entertaining” them leads to putting them into action.
I’m not sure why billorites would extrapolate from President Carter’s mentioning the *sin* of lust to the proposition that sex outside marriage is just fine. (Admittedly, Carter went bats, but ...) Sin is sin. Whether an attitude is lust, or just (ahem) admiration for a gentleman’s intellect and style, is something the individual has to address personally.