Things are not right and wrong because of any law. They are right and wrong because they are simply by their very nature right or wrong. They are natrually right and wrong. The ten commandments did not cause things to be wrong, they only helped guide the Jews into recognizing them as being wrong.
You see, God made man in a state of grace. Man did not know sin, they did not know right and wrong. They only knew right. Imagine a 5 year old. Biological issues aside, a 5 year old could not commit fornication for the very reason that she wouldn’t even know such a thing existed. Adam and Eve were like this to sin more or less. Then, Adam and Eve ate of the fruit of the tree of knowledge of good and evil, and just like the serpent said, they became like God. Satan is so devious, he can lie by telling the truth, and he did. They did become like God because now they knew sin. Once they knew it, their priorities became confused (we call it concupiscience), and some sinful things started to take priority in the life of man over things like God.
As a guide, God gave the ten commandments to the people so that they would have a clear indication of what was right and wrong. The ten commandments pointed out what was right and wrong to the Jews, but the things were right or wrong independant of the commandments. Adultery was always wrong, it didn’t just become wrong when God gave the commandment. A great deal of what Paul writes about in Romans 6,7, and 8 is to make this very point.
The rest of the law, the ritual law (which is what most people call the Mosaic Law) was only given after the Jews sinned by worshipping the golden calf. The Levitical law did make things right and wrong just because it was given. It was designed to really hammer obdedience into the minds of the Jews. It was also designed to be so complicated and overwhelming that no Jew could follow it so that they would realize their need for a savior.
The reason we follow the ten commandments is because they are still a wonderful guide to sum up the basic moral precepts that are simply naturally right and wrong. All of these things were always right and wrong, even before the law was given. For instance, Cain is guilty of killing Abel, even though there was no law that pointed it out at the time. There are other sins that we see committed in the book of Genesis, too, even though the ten commandments had not been given yet.