forbidden fruit
Noun
Meaning
originally an apple from the tree of knowledge of good and evil in the Garden of Eden; it is now used to refer to anything that is tempting but dangerous (as sexuality)
Source: WordNetHypernyms
Examples
You got to set your mind right and the rest will come to you naturally. No restrictions, no hang-ups, no stupid rules, no formalities, no forbidden fruit - just everyone getting and giving as much as he and she can.
Money is coined liberty, and so it is ten times dearer to the man who is deprived of freedom. If money is jingling in his pocket, he is half consoled, even though he cannot spend it. But money can always and everywhere be spent, and, moreover, forbidden fruit is sweetest of all.
But money can always and everywhere be spent, and, moreover, forbidden fruit is sweetest of all.
Forbidden fruit tastes best.
Forbidden fruit is sweetest.
Away from home the girl picks forbidden fruit.