There are many covenants, but two major covenants are the OLD and the NEW. There were many things that was changed from the old covenant to that of the new covenant.
old: Can't eat pork, alright to hate your enemies, alright to stone a person to death, alright to sacrifice animals to forgive your sins, obey all the laws, commandments, statutes, ordinances or your going to Hell, actually have to commit adultery to be an adulterer.
new: Can eat pork, have to love your enemies, can't stone anyone to death, can't sacrifice animals to have sins forgiven, only have to obey what Jesus and His Apostle taught, namely LOVE ONE ANOTHER, and looking at a woman to lust after her you have committed adultery. There are a few examples off the top of my head, there are many more changes made between the old covenant and the new covenant.
Before any covenant, ALL things were permissible with God, even incest. Once there was enough people on the Earth, and God understanding all things, and understanding how the genetic code deteriorates with each generation, then made it against the law to practice incest. But a day is coming where brother and sister marrying will be accepted.
True, and technically, no where does it say they did not.
The other humans you are referring to, were not humans at all, but demons taking on Earthly flesh of different kinds. When these things would have sexual relations with human women, it created the giants, and the nephilim in those days. But all humankind came from Adam and Eve. Now at the time of Adam and Eve, there were other kinds here, not Humankind, but other kind, satan and billions of demons were here as well, and they do not like the abyss, the abiding in nothingness, they indwell physical things, being animals, birds, plants, rocks, whatever, as long as they were IN something, possessing something.
I just prefer consistency from God, not a God who green-lighted incest at first and then prohibited it later on. Its confusion, and difficult to fathom how something could be right and then become wrong. Perhaps God flip-flopped on gay marriage too? 
Search Scriptures or Hebrew and Greek? Scriptures does not teach there were other men and women in the days of Adam and Eve, Yes there were others there, but Scriptures does not teach those others were human men and women, that is your assumption of what the "others" were.
There is the problem, Only God interprets. When men started interpreting the Bible for themselves is when all manner of false doctrines and false teachings came into existence. Interpretations belong to God NOT TO MEN.
^i^ Responding to Post #32