Free said:
So were rape, murder, coveting, homosexuality, sacrificing children or bestiality wrong only after God said they were wrong?
I'd like to reply by repeating my earlier question, "Why is marrying a close relative immoral?". We all marry relatives, albeit distant ones. Abraham married his half-sister (albeit as a pagan!). Isaac married his near cousin, as did Jacob. Going further back in history, the children of Noah's kids Shem, Ham, and Japheth must have also married each other (whether direct siblings or near cousins), as they "restarted" humanity after the Flood wiped everyone else out.
The answer is still "Because God says so" (whether by actual spoken decree or not - we go by His "opinion" on a matter, He is the standard), is it not? Out of who He is, His character, His will, His nature, His decrees, we obtain the standard for morality.
The examples quoted above always were and always will be wrong. For example, that murder was a sin from the beginning, before God wrote "You shall not murder" on the tablets of stone for Israel, can be seen in God's response to Cain killing Abel. Why are they wrong? Well, for one, they pervert the natural order which God has ordained.
But the question of Cain's wife is slightly different. By ordaining that Adam and Eve be the sole progenitors of the entire human race, God in effect decreed that the first generations would have to marry very close relatives. Such marriages were thus entirely
within the natural order God had made. Providing they were faithful, exclusive, one-man-for-one-woman marriages, there was nothing intrinsically wrong about relationships between close relatives at that time.
The argument from genetics as a plausible reason for why God later made the decree against close-relative marriages is simply a reasonable deduction from what we now know through scientific discovery. He knew the risks such relationships would now pose, and thus moved to maintain the purity of His chosen people, through whom ultimately the Messiah would come. The same is true for much of the Law - there was nothing intrinsically wrong about eating certain animals, for example - as can be seen by our freedom to eat them now - but for every Israelite living at that time eating those animals was most certainly wrong, because God decreed it so. We can deduce reasons for even those laws, too. Of course there is the spiritual significance and foreshadowing of Christ, but there are also many practical reasons that can be suggested, such as the risk of disease (e.g. from pig meat) and so on. God was ordaining a code by which the Israelites would remain distinct and untainted, that they might be a light to the nations and bring the Messiah into the world.
The problem with not accepting that Cain married a near relative (whether sister, neice, etc) is that the Bible is emphatic that Adam and Eve were the first humans on Earth; their offspring make up all of humanity. The only possible deduction, therefore, is that Cain married a close relative. He had no other choice! I agree that this raises questions that we may find hard to grapple with, but it's a lot safer than denying the clear teaching of Scripture that Adam and Eve were the first humans.