The Bible does not say what the fruit was, but whatever it was, it was the only forbidden thing on earth.

Gen.1:26–30 “Then God said: “Let us make man in our image, according to our likeness, and let them have in subjection the fish of the sea and the flying creatures of the heavens and the domestic animals and all the earth and every creeping animal that is moving on the earth.” 27 And God went on to create the man in his image, in God’s image he created him, male and female he created them. 28 Further, God blessed them, and God said to them: “Be fruitful and become many, fill the earth and subdue it, and have in subjection the fish of the sea and the flying creatures of the heavens and every living creature that is moving on the earth.”

29 “Then God said: “Here I have given to you every seed-bearing plant that is on the entire earth and every tree with seed-bearing fruit. Let them serve as food for you. 30 And to every wild animal of the earth and to every flying creature of the heavens and to everything moving on the earth in which there is life, I have given all green vegetation for food.” And it was so.” NWT WOL.

Verse 28 disproves the claim that sex and reproduction was forbidden, rather Adam and Eve were specifically told to reproduce with God’s blessing.

The prohibition was stated in Gen. 2:15–17. “Jehovah God took the man and settled him in the garden of Eʹden to cultivate it and to take care of it. 16 Jehovah God also gave this command to the man: “From every tree of the garden you may eat to satisfaction. 17 But as for the tree of the knowledge of good and bad, you must not eat from it, for in the day you eat from it you will certainly die.”