The other thing about OT incest--sure, if there is a situation where there are only two people left in the world, a man and a woman, it's difficult to repopulate when you're short on people! I don't see what it would be unjustified in that situation simply because you don't really have any better choice.
What always confuses me, though, is Genesis 1, where God says "Let's make man in our image," and then it proceeds to say that "God made man in his own image; in His own likeness He created him; male and female, etc., etc." Adam doesn't even show up until chapter 2. So what was it? Was Adam rather a special representative of man, which is what the name "Adam" means if I understand correctly, specially created by God for that purpose and thus bringing sin on all of creation in his fall from glory? If so, saying Adam is the "first man" is nothing other than referring to the special nature of Adam's creation for the high purpose God called him to. He is the first named man of the Bible. He is also the ancestor of all now living since everyone else except his descendent Noah died in the deluge. That would suggest that there were predecessors to human beings before Adam and that his sons and daughters mixed with them post-Eden. For example, it is written that Cain built a city and named it after his son. You can't build a city without people to populate it, and you can't become a great leader if there's no one around to lead. If human beings already existed pre-fall and given the commandment to "be fruitful, multiply, fill the earth, and subdue it," then there's no need to assume any incest EVER took place.
Personally, I identify as a conservative, mainline, Protestant evangelical and my peers would probably call me a heretic for pointing this out. Whether humans existed prior to Adam in no way challenges or changes doctrine. What's plain is that the patriarchs somehow knew what was expected of them even though the law had not yet been written. Obviously there was already an oral tradition in place. Even Cain and Abel brought regular sacrifices, something they'd have been taught by their own parents to do. So if they were aware of how to worship God, then they'd have been instructed against incest.
The typical justification in favor of patriarchal incest is what has already been mentioned, that the DNA had not yet been corrupted and thus no reason NOT to father children through a sibling or a cousin, assuming Adam and Eve to be the only two humans alive after the creation. I don't see this as inherently wrong, either. The law of Moses changes this, reflecting a new stage in human development. The Bible doesn't speak well of Egyptian/Israelite relations, since Egyptians did not like hairy, smelly herdsmen. The Egyptian nobility had a tradition of sibling unions, and avoiding such in the future, i.e. after the patriarchal period, would function to separate Israel from the practices of other nations deemed by God to be wicked.