forbidden fruit

Fig. someone or something that one finds attractive or desirable partly because having the person or thing is immoral or illegal. (Biblical; from the apple in the Garden of Eden that was forbidden to Adam by God.) Jim flirts with his sister-in-law only because she's forbidden fruit. The boy watches that program only when his parents are out. It's forbidden fruit.

something that you want very much but are not allowed to have, especially a sexual relationship He'd spent many years lusting after his brother's wife - the forbidden fruit.

Unlawful pleasure or enjoyment; illicit love. For example, After Mary moved in with John, Tom began courting her-forbidden fruit is sweet, I guess , or Smoking behind the woodshed, that's a case of forbidden fruit. This expression alludes to Adam and Eve's violation of God's commandment not to touch fruit from the tree of knowledge, which resulted in their expulsion from the Garden of Eden (Genesis 3:6). In the form forbidden fruit is sweet it appeared in numerous early English proverb collections.