In the first place, the Bible does not teach that Adam and Eve were forbidden to eat an apple. The Bible says that God told them not to eat the fruit of the tree. I suggest that you look carefully at Genesis chapter 2, from verse 8-9 and then 16-17.
“Now the LORD God had planted a garden in the east, in Eden; and there he put the man he had formed. 9 The LORD God made all kinds of trees grow out of the ground — trees that were pleasing to the eye and good for food. In the middle of the garden were the tree of life and the tree of the knowledge of good and evil.
16. And the LORD God commanded the man, “You are free to eat from any tree in the garden; 17 but you must not eat from the tree of the knowledge of good and evil, for when you eat from it you will certainly die.”
No mention is made that it was an apple tree. In Genesis chapter 3, when the serpent approached Eve, nothing is mentioned about an apple. Please see chapter 3.
“Now the serpent was more crafty than any of the wild animals the LORD God had made. He said to the woman, “Did God really say, ‘You must not eat from any tree in the garden’?”
2. The woman said to the serpent, “We may eat fruit from the trees in the garden, 3 but God did say, ‘You must not eat fruit from the tree that is in the middle of the garden, and you must not touch it, or you will die.
“You will not certainly die,” the serpent said to the woman. 5. “For God knows that when you eat from it your eyes will be opened, and you will be like God, knowing good and evil.”
6. When the woman saw that the fruit of the tree was good for food and pleasing to the eye, and also desirable for gaining wisdom, she took some and ate it. She also gave some to her husband, who was with her, and he ate it.”
It was a fruit but we do not know what type of fruit it was, but it was definitely not sex. Therefore, your grandfather and many who teach that Eve had sex with the serpent are wrong. And if you go out and preach that the serpent and Eve had sex in the garden, you will be teaching something contrary to the scripture. I can say more but because of space, I have to stop there.