When Adam and Eve were in the garden of Eden they were not mortal. Even though they had physical bodies, they did not know pleasure or pain, good or evil and were not subject to death.
They had been given a commandment not to eat of the tree of knowledge of good and evil because if they did, they would die. Eve knew that this would be the only way for them to progress. Satan wanted to tempt them to break the law but he did not know how that would fit into God's plan.
Some Bibles refer to the Fall as "shameful". As Latter-Day Saints, we know better. It was necessary. Dallin H. Oaks explains the difference between transgressing a law and committing a sin. Sins are inherently wrong. Murder is a sin. Operating without a license is wrong because it is against the law but it is not a sin. Eve transgressed the law but did not commit sin.
Because Adam and Eve partook of the forbidden fruit, they were cast out of the garden and thus their bodies experienced a change. They became mortal. Only then were they capable of procreating. They opened the portal by which all mankind would come into mortality and experience earth life. If it wasn't for their transgression of that one law, none of us would be here.