I would just like to add to what Alpha has just eloquently said. Adam and Eve were the first humans to begin the human race. If God had created other humans they would have been without sin like Adam and Eve first were. But all have sinned, the Bible says, and we trace this to Adam, meaning no others were invloved seperate from Him in the dispersing of sin to us.
For the Bible says that Jesus was the Last Adam. The first Adam brought sin on the entire human race by birth. Adam #1 is the origin of our sin. He's the father of our sin. Thus the everlasting father of sinners. The last Adam (Jesus) came to deliver us of our sins, thus free us from Adam. Jesus becomes ,subsequently, as the scriptures prophesy in Isaiah 9:6, the everlasting father (of the new or born again race of Jesus, the righteous).
The destructive nature of incest was not immedeiately felt in the kingdom of humanity. It could later, to man's possible extinction. God could not allow this. To fulfill His promise in Genesis to deliver man he would introduce rules for relationships and marriage so as to secure His place in time for His coming. Other reasons as well. This is one.
DMP