Very good. Were the Ten Commandments the first time the Hebrews ever heard of those laws? Were they lawful prior to Moses? Few believers have ever considered that question.
One must either concede that laws were given to Adam that aren’t in the text, or accept the God is unjust in meting out punishment to those ignorant of His desires.
Deut. 32:7 Remember the days of old, consider the years of many generations; ask thy father, and he will declare unto thee, thine elders, and they will tell thee.
8 When the Most High gave to the nations their inheritance, when He separated the children of men, He set the borders of the peoples according to the reckoning of the children of Israel.
9 For the portion of the LORD is His people, Jacob the lot of His inheritance.
Look back. What did Jews of old believe? What do acknowledged teachers say that is consistent with ancient understandings?
Also see:
http://www.jewishpath.com/parsha/bereishis/the_seven_noach_commands.html