You are right. According to the Fathers of the Church and St. Thomas, Adam and Eve did not have intercourse until after the fall. There are precise theological and moral reasons for this teaching, too complex to go into here. The original sin was an act of disobedience followed by Adam’s ignoble blaming of Eve for his sin. Whatever eating the fruit (it was not an apple that was depicted in Genesis) means, it does not mean having intercourse, because that is something God commanded them to do, not something he forbade them to do. The first sin was doing something forbidden, not doing something commanded. When you reflect on this you see how incoherent this opinion is.